add and remove invitees on an event

[grounded042]

We need to let an event admin add and remove invitees from an event. Removing an invitee is done by deleting them from the system. This functionality needs to exist with auth. We will need the following routes:

• POST /events/:id/relationships/invitees - create an invitee for the specified event
• DELETE /invitees/:id - delete an invitee based on their id