Added option to specify group types for nollning events

[hannesblaman]

Added two options for nollning events (will need database migration):

• mentor_group_types: List specifying which types of group are allowed for an event (mentor, mission, committee and/or default)
• allow_other_mentors: Not sure if this is strictly needed, but thought it could be useful for events like nollegasque.

Added validation when sending signup request (including signup edit) and new endpoints for getting relevant information for when a user signs up.

Also fixed a bug where the lottery option was not part of EventCreate schema.

Corresponding PRs have been made in frontend and app repos:

• Complete revamp of event creation UI (including option to specify group types for nollning events) WWW-Web#318
• Added nollning event group types App2#327

[georgelgeback]

Please read the comment. I fixed some bugs I managed to find which didn't require any judgement calls, and added unit tests because I like them.

I was in the room when a previous spindelman decided not to give admins the ability to change the lottery field after an event has been created, but I'm not sure why this was. Did some searching and found no reasons in the backend nor frontend so I think it's fine to add it. Just in case I added an extra guard so it cannot be edited after spots have been confirmed (it doesn't make sense to switch to/from lottery then anyway).

Also have a look at the new unit tests I created. test_events is unrelated to these changes (but I felt it was important to have them), but test_event_signup includes some judgement calls so you might just want to have a look over what the tests do and edit them if they are not testing for the correct things. Apologies for making the PR 4x as large in LOC.

Other than that, LGTM 👍

[georgelgeback]

Was there meant to be code here which you forgot to push, or is it a remnant and meant to be removed?