Closes #6749
Closes #8321
Closes #9374
Closes #2700
Closes #2639
Closes #2355 
Closes #1861
Closes #8757
Closes #7228
Closes #10870
Closes #6193 
Closes #11299
Closes #11468
Closes #9317
Closes #11300 (will incorporate a fix to this PR's issue)
Closes #11046 (will incorporate a fix to this PR's issue)
Contributes to #5944 
Contributes to #11475
_...and possibly more!_

This PR makes deleting messages (automatically and manually) a lot easier on Rocket.Chat.

- [X] Implement a bulk message deletion notification, to quickly push large message deletions to users without reload
  - [X] Use it in `rooms.cleanHistory`
  - [X] Use it in user deletions
- [X] Completely remove cleanChannelHistory as required by v0.67
  - [X] Remove server method `cleanChannelHistory`
  - [X] Remove REST API `channels.cleanHistory`
- [x] Implement a sidebar option to clean history
  - [x] Basic implementation
  - [x] Allow excluding pinned messages
  - [x] Allow attachment-only mode
  - [x] Allow specifying user(s) to narrow down to
    - [x] Also update REST API
    - [x] Also update docs
  - [x] Break the deletion into multiple different requests, so the client can keep track of progress
  - [x] Clear animation / progress bar for deleting
- [x] Retention policy
  - [X] Global, set by admin
    - [X] Global timer that runs every second and deletes messages over the set limit
      - [X] Can change its timer's resolution to prevent insane CPU overhead
    - [X] Admin can decide what room types to target (channels, groups and/or DMs)
    - [X] Allow excluding pinned messages
    - [X] Allow attachment-only mode
  - [x] Per-channel, set by those with a new permission
    - [x] Disabled when master switch off
    - [x] Set in channel info
    - [x] Can override global policy with a switch that requires `edit-privileged-setting`
    - [x] Allow excluding pinned messages
    - [x] Allow attachment-only mode
    - [x] Uses same global timer for cleanup
  - [X] Message at start of channel history / in channel info if there is a retention policy set
  - [x] Message in channel info if there is a retention policy set on that channel specifically
- [X] Make cleaning history also delete files (completely!)
  - [X] Manual purging
  - [X] Automatic purging
- [x] Make other deletions also delete files
  - [x] User deletion
    - [X] Own messages
    - [x] DMs with them's partner messages
  - [x] Room deletion
- [x] Cleanup
- [x] Finish related [docs](https://github.com/RocketChat/docs/pull/815)
- [x] Link to the docs in the settings

Please suggest any cool changes/additions! Any support is greatly appreciated.

**Breaking change:** This PR removes REST API endpoint `channels.cleanHistory` and Meteor callable `cleanChannelHistory` as per the protocol specified for them.

![bzzzzzzzz](https://user-images.githubusercontent.com/39674991/41799087-56d1dea0-7670-11e8-94c0-bc534b1f832d.png)