Add event sign in

[mirdaki]

We want to be able to record attendance for events. We'll have a sign in on the club website that will call this REST API. I posted about it on the club website, but I'll include the backend's view of the use case here.

1. Receive a sign in with a UFL email
2. Find what event would be happening (plus/minus an hour or so). If no event is happening, return an error saying so. If the email is invalid, return an error saying so.
3. Add that member to the attended list for the event
4. Check if the member is an existing member. If they aren't, send a success message with a request for more information. If they are, just send a success message.
5. If the person is a new member, they may or may not choose to provide more information. That call will go to the "add new member" use case Add new member #5. We need to discuss how we want to decide how long a new member can provide information about themselves.