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.
Well, technically the name of the variable is not important at all, it's where the file is located on disk that matters. Because when you search (in the route below) you specify the singular form, that is where the resolver assumes it to be. It is possible to buck the system here, but likely not worth the fight. 

Generally speaking the models use the singular form.