[WIP] [3.2][feature/revisions] Implement post revision tracking

The majority of the functionality has been tested and works. There are still a few minor things that need to be done and tested, but I would like to go ahead and get the peer testing/reviewing started.

To test:
- Install this branch
- Go to the Post Settings module in the main ACP tab, scroll down to the last category of settings, and ensure the feature is enabled. All other settings added by this PR can be left as default unless you wish to test a specific setting.
- Ensure that your account has moderator permissions for viewing and managing post revisions, or at least that you can view your own posts' revisions.
- Edit a post at least once (if you're testing the user f_revisions permission, it has to be one of the user's own posts or a wiki post).
- When viewing the post, click "View edit history". When javascript is enabled, this will review a short list of the latest revisions directly beneath the post. If you click the "View edit history" link in the "LAST 5 REVISIONS (VIEW EDIT HISTORY)" line, or if javascript is disabled, it will take you to the full revisions view page.
- Play around with it some and let me know what does and doesn't work, and what needs to be changed.

Still to do:
* Testing
- [x] Wiki Posts functionality
- [x] Permissions
- [x] Automated tests (unit, functional)
* Other
- [x] Create migrations (instead of database_update.php changes)