[MERGEABLE] Splitview With Image and Custom View Support

https://trello.com/c/A22yIIal/1282-splitview-add-view-provider-mechanics

This PR is can be merged into jeff/splitview-1x2 when ready
* Adds a MainViewFactory for view factories
* Adds support for images
* Adds support for extensible custom views
* Adds support for 1-off views
* Adds a view state cache singleton
* Fixes #6356

I'm in the process of converting all of our jQuery Deferred/Promise objects to ES6 Promise objects, so you should get in the habit of using `.then(resolved, rejected)` instead of `.done()` and `.fail()`