CLC-4366 Move rails_admin into a Rails engine with its own gem space

First part of https://jira.ets.berkeley.edu/jira/browse/CLC-4366

This just injects rails_admin into our app via the "ccadmin" Rails engine, so that ccadmin can have its own gem space. Everything works properly in both regular dev and compiled-assets mode. 

I haven't yet done the hard part, which is having 1 version of font-awesome for rails_admin and another version for the main app. I'll need help from @christianv  to accomplish and test that scenario.