Add route for looking up users by dataset name

- Doing name for now, not data-group/type, as that seems to be the way we're implementing our routes at the moment and I didn't want to get into that refactor/addition in the scope of this story
- Implementation: `data-sets/some-data-set/users` brings back the users assigned to `some-data-set`
Stub method for doing user lookups by passing data-set name
Return a `user_api_object`

- At the moment this will just be an email, but I thought wrapping up an api object in the model might serve as a good guide for passing back a partial version of the backdrop user object
- Still using name
Merge pull request #176 from alphagov/dataset-ownaz

Add a view for looking up users by data-sets