CLC-4366 Reverting rails_admin engineification

Reverses #3062  for https://jira.ets.berkeley.edu/jira/browse/CLC-4366 until we can rethink the approach. 
@ctweney seems like we should also remove this from the `Gemfile.lock`:

```
PATH
  remote: ccadmin
  specs:
    ccadmin (0.6.5)
      bootstrap-sass (~> 3.2.0.2)
      rails (~> 4.1.6)
      rails_admin (= 0.6.5)
```