OR-738 constantize the "invitation" and "request" membership types
OR-738 refactor basic checks in community controller
OR-738 forward community invitation workflow
OR-738 split DELETE /api/communities/:id/membership/:user_id into 4 cases

Each case emits its own event.
OR-738 handle usernotification tasks

* when a manager cancels an invitation, removes the attendee usernotification
* when a manager refuses a request, add a usernotification for the attendee
OR-738 fix doc, formatting and useless return false statement
Merge pull request #534 in OR/rse from ~MBAILLY/rse:OR-738-augment-delete-endpoint-invitation-membership to master

* commit '320657ed8a43c1565581bd69663c9dff28ec82b8':
  OR-738 fix doc, formatting and useless return false statement
  OR-738 handle usernotification tasks
  OR-738 split DELETE /api/communities/:id/membership/:user_id into 4 cases
  OR-738 forward community invitation workflow
  OR-738 refactor basic checks in community controller
  OR-738 constantize the "invitation" and "request" membership types