Make activeadmin-mongoid play nice with ActiveRecord Resources

I am developing a Rails application utilising both ActiveRecord and Mongoid.
I use ActiveAdmin and activeadmin-mongoid.

Since activeadmin-mongoid directly monkeypatches ActiveAdmin, it destroys functionality for ActiveRecord-based resources.

This patch first checks whether the resource is Mongoid-based, and if not just calls the original ActiveAdmin implementation.

It adds the necessity to add
```ruby
before_filter { @skip_sidebar = true }
```
to your mongoid-based resources.