Pull resource methods into their own controllers

[sipple]

This needs to not be happening: https://github.com/pgharts/trusty-cms/blob/master/app/controllers/admin/resource_controller.rb#L58-L65

Currently, all resources methods in admin go through one controller, where a class_eval handles all the work of showing, updating, creating, etc. This makes it impossible to do things without mass assignment, and it's also just not great API design.

Pulling these out into their own controllers is a chunk of work but, ultimately, worth it. The reason I don't want to do this yet is because I'm not sure how this will impact extensions and will need a better handle on that before I go about mucking with this.

After Snippets and Assets are integrated, we can make a decision from there.

[sipple]

More information on this:

All resource controllers in Admin inherit from ResourceController, so some override ResourceController's create/update methods with their own, and some do not. That makes fixing this a bit easier, as we can pick at them one at a time without rewiring everything, and without breaking existing functionality. I might tackle this for Users for #91 to see how it works.