Actually closes #1579, likely closes #544, definitely closes #1670

This PR builds on the work done in #1584 by preventing the buttons for creating a new comment thread or replying in an existent one from showing up if the user can't comment (not only when it's impossible to comment [specifically, when the user is not a mod or an admin & comments on the post are disabled, or it is locked or deleted]).

Most importantly, it adds 2 new site settings for rate-limiting comments:

• RL_NewUserCommentsOwnPosts for new user comments on their own posts;
• RL_CommentsOwnPosts for unrestricted user comments on their own posts;

Combined with the existing settings, users will now be limited to the following number of comments per day (by default):

• new users: 0 comments on posts of others, 30 on their own posts & answers to them;
• unrestricted users: 50 comments on posts of others, 50 on their own posts & answers to them;

Open to discussion on whether to adjust the number of comments users can make on their own posts per day.

The PR also ensures that the add thread / reply buttons are always shown to users but disabled if they can't comment for one reason or the other. The exact reason as to why a user can't comment is moved to the button's title (instead of littering the page with huge notices):

It also makes possible to take actions on inline comment threads (comment editing & deleting included):

Plus, locked threads in post comment threads lists now show the lock icon (as do other "restricted" states):

We also no longer display the "Showing X of Y comments. See the whole thread." widget if X is equal to Y (it simply wastes space in such cases):

It also adds several scopes (and subsequently increases coverage across the project to make sure the new scopes don't break anything important):

model scopes

• AuditLog.of_type(name) (only include logs of a specific type by name);
• Comment.by(user) (only include ones made by the user);
• Flag.by(user) (same as Comment.by);
• Flag.declined (only include those that are handled as declined);
• Flag.helpful (only include those that are handled as helpful);
• ModWarning.active (only include warnings that are currently active);
• ModWarning.to(user) (only include warnings issued to the user);
• Post.bad (only include posts with score < 0.5);
• Post.by(user) (same as Comment.by);
• Post.good (only include posts with score > 0.5);
• Post.in(category) (only include posts made in the category);
• Post.on(community) (only include posts made on the community);
• Post.parent_by(user) (only include posts that have a parent made by the user);
• Post.problematic (only include posts with score < 0.25 or that are deleted);
• PostHistory.by (same as Comment.by);
• PostHistory.of_type(name) (same as AuditLog.of_type);
• PostHistory.on_undeleted (only include events made on undeleted posts);
• SuggestedEdit.approved (only include those that are decided as approved);
• SuggestedEdit.by(user) (same as Comment.by);
• SuggestedEdit.rejected (only include those that are decided as rejected);
• Vote.by(user) (same as Comment.by);
• Vote.for(user) (only include those where the target is the user);

concern (shared) scopes

• Lockable.locked (only include ones that are locked);
• Lockable.unlocked (the inverse of Lockable.locked);
• SoftDeletable.deleted (only include ones that are marked as deleted);
• SortDeletable.undeleted (the inverse of SoftDeletable.deleted);
• Timestamped.newest_first (order by created_at DESC);
• Timestamped.oldest_first (order by created_at ASC);
• Timestamped.recent (only include ones made in the last 24 hours);

as well as removes some existing scopes:

• CommunityUser.active (replaced by CommunityUser.undeleted to align with other soft-deletable models);
• User.active (replaced by User.undeleted to align with other soft-deletable models);

It also removes check_edits_limit! as it's been unused since 412b490 (the check has been done inline for at least 4 years - we might want to abstract it into a similar helper).

Additionally, it adds a QoL []= class method to the SiteSetting model for us to be able to update site settings simply via SiteSetting[name] = value without having to explicitly handle value type conversion & cache updates.

Tangentially, it also adds two new methods:

• max_votes_per_day that functions identically to max_comments_per_day (proper logic encapsulation);
• recent_votes_count that functions similarly to recent_comments_count (unsure why we exclude self votes on parents but not votes on own posts there);

as well as renames some existing ones:

• User#can_push_to_network to can_push_to_network? (to align with the naming convention for boolean checks);
• User#can_update to can_update? (same as with can_push_to_network);
• User#can_see_deleted? to can_see_deleted_posts? (the check only applies to posts);
• User#has_ability_on to ability_on? (same as can_push_to_network);
• User#has_profile_on to profile_on? (same as can_push_to_network);
• User#is_moderator_on to moderator_on? (same as can_push_to_network);
• ModeratorController#verify_can_see_deleted to verify_can_see_deleted_posts (same as with can_see_deleted?);

and a couple of shared concerns:

• Inspectable that provides a generic inspect method that can be used on any model;
• Lockable for logic shared by all models representing records that can be locked;
• SoftDeletable for logic shared by all models representing soft-deletable records;
• Timestamped for logic shared by all models representing timestamped records;
• UserRateLimits that houses methods related to various rate limits (f.e., comments & votes per day);
• UsernameValidations that encapsulates username-related validations (user model is too large);

Some extras:

• Layout/SpaceInsideArrayLiteralBrackets is now enabled to match our style guide for Ruby;
• assert_json_response_message(expected) test helper method for asserting that response body contains a given message;
• assert_redirected_to_sign_in test helper method for asserting that response has the :found HTTP status code & redirects the requestor to sign in;

Related meta issue: meta:294088