removed unloadable from controller

Unloadable causes issues while running in Rails 4.

Removed unloadable from the controller because of a RuntimeError (Circular dependency detected while autoloading constant EmailPreviewController), which happens due to changes of unloadable from Rails 3 to Rails 4. The problem is discussed here: https://github.com/thoughtbot/clearance/issues/276.  Let me know if this solution doesn't work for you!