Remove user key/permittable params, change to use id

@FWKamil quite an opinionated set of changes ;)

  * Further focused the gem by removing permit table user params (know this was my idea, but I now think this should be done inside the implementing App and not as the responsibility of this gem).
  * Remove user_key and just use the id column on the user table. This will improve speed on large DBs as its now an integer. I couldn't think of a downside.
  * Controllers use current_notifiable_user, so authentication is pushed onto the implementing App. If no authentication is required then you can just return nil, which should work for The Open.
  * Small changes to destroy behaviour - now actually deletes the DeviceToken.
  * Some updates and fixes to API methods and return codes.

Let me know what you think. Happy to chat through with you on Monday.