Refactoring to override finder

* I want to have a different short link finder method so moving that
piece into a before_filter so I can override it in another module
WIP: Override Finder method in controller

* Rather than find a url by unique_key, also see if the domain matches
the request, this will allow multiple domains to be served by 1
application using this Rails Engine
* Note - this is not currently overriding
Shortener::ShortenedUrlsController#set_shortened_url - still needs to be
investigated