Wip mongoid history

@niedfelj 

Ready for a look.  'History' subtitle on a content edit page (content logs) is now clickable and goes to the view for its history.

Issues: 
* For some reason, I can’t associate a history object with a user, getting “uninitialized constant User." I’ve tried ``belongs_to :modifier, class: ‘ClearCMS::User’`` and some other stuff, but can’t seem to get the namespacing to work.  This happens in both the automatic history tracking for a model, as well as the explicit history tracking like tracker.undo! or .redo! where it manually accepts a user param.   Thoughts?
* (Because of that, contents_controller is a little messy right now)
* Undo deleting an image works nicely but redo creating one (i.e., duplicating) does not, and uses fallback. (Because the asset create method acts as if something has just been uploaded to s3 and is passing URL)

Sublime took out a lot of spacing, might wanna use ?w=1 on diff.  Thanks.