Update the ModelBridge to remove deprecated finders and support Mongoid

The `find_by_login`-type finders have been deprecated and will be removed shortly. This updates the ModelBridge to no longer rely on them. 

The approach taken is compatible with Mongoid, so we can now support Mongoid. This has been tested with Mongoid 4, but only uses functionality from 3.1.0, so it should be fine with older releases too.