fURL managers can requests a furl

This first version offers a form where publishers can request new friendly urls. Although these requests are persisted, nothing is done with them yet other than sending a notification to fURL managers with the relevant information.

Story: https://www.pivotaltracker.com/story/show/74519812
Gem versions should be pinned as per the styleguide: https://github.com/alphagov/styleguides/blob/master/ruby.md#gemfile