Closes (when #7130 is merged): #7179
Closes (when #7130 is merged): #2544
Related: #6257, #6882

Note: Merges to fix/autosave-preview (#7130) as base branch.
Note: As best I can tell, this branch is fully functional, but I have plans to include new end-to-end tests for various preview circumstances prior to merge.

This pull request seeks to allow the user to preview changes to a published post without first updating the post (i.e. the autosave revision preview).

In doing so, it...

• Refactors the PostPreviewButton component to avoid relying on a specific save workflow, and instead leverage the mere availability of a preview link
• Removes preview_link from the posts endpoints and adds instead to the autosaves endpoint (see REST API: Autosaves entities should include preview_link #7179)

To the latter point, I landed at the conclusion that preview_link only makes sense in the context of autsosaves on consideration of:

• The post endpoints reflect the value of the canonical saved post. If there are no unsaved changes, the user should be directed to post.link as its preview.
• It does not belong in the revisions endpoints because it is not the case that any revision can be previewed. A preview only occurs for the latest revision of a post (_set_preview, wp_get_post_autosave).

Related Trac tickets:

• REST API: Support autosaves (43316)
• REST API: Expose 'preview_link' for viewable post types (44180)

Thus, if there are no unsaved changes for a post, the Preview button is effectively a link to post.link. If there are unsaved changes, the autosave is triggered and once its responses' autosave.preview_link becomes accessible, it is used in the redirect (regardless of whether or not the autosave occurred as a full save as described in #7124).