Use of slugs in importing Requests #60841260

Per [story 60841260](https://www.pivotaltracker.com/story/show/60841260).  Had to override Sluggable mixin to make `title` nullable.  Also need to change the default behavior for assignee: if the uploader does not give one for an object that already exists (identified by slug), then it defaults to the existing objects's assignee value (and notifies user of such) *before* defaulting to audit contact.