WIP: add ability to hide specific post revisions

This is a start for the feature described here:
https://meta.discourse.org/t/ability-to-hide-or-delete-old-revisions-on-selected-posts/19389

Changes so far:
1. PostRevision only looks at the left-hand side of a given record.
2. Added methods to hide! (or later show!) a given revision
3. A little bit of refactoring so all fields use current(field) and previous(field) methods
4. Fallbacks to previous/next revision or post, depending on what is available / not hidden

I haven't looked too closely yet at where to expose the methods via a controller or how to handle the ember side of things.

Also, both the left and right hand side are still saved to the database right now.

For starters, I just want to see if this approach is agreeable or if there's any other input.