Convert general settings page to ember data

Issue #2846
-Implement custom RESTAdapter and serializer for settings data.
-Convert theme selector to Ember.Select.
-Finish upload modal and wire into settings page.
Because with the model, adapter, and serializer set to plural (or various combinations of singular and plural) eventually the Ember resolver stops being able to figure out what it needs to find.  Believe me, I tried, but eventually just went with all singular entity names because it worked.  There's probably a way to get it to work but at a certain point it's just better to just :shipit: