Apply filters from ActiveAdmin config to overridden Devise controllers

I'm not thoroughly familiar with some of the meta-methods in ActiveAdmin, so I'm not 100% positive there aren't unintended side effects from this (e.g. potential circular filters like requiring login on a login page), but we're currently using it and everything seems to be working properly.

AA allows you to write global before_filters (and after and around, etc.) for all AA controllers in the initializer like this:

```
  config.before_filter :do_something_awesome
```

The problem is, they're only added to `ActiveAdmin::BaseController`. But AA also has overridden Devise controllers for Sessions, Passwords, and Unlocks in `ActiveAdmin::Devise`, with no way of ensuring the specified filters are added there as well. So, I made sure that the filters get applied to those overridden controllers.