Fix none nollning group test fails

[hannesblaman]

Updated one test, added one test and fixed the event signup code to pass the last test

[georgelgeback]

Do we want to allow someone to:

1. Sign up to a nollning event with a certain group
2. Edit their signup so that they don't have any group associated with the sign up

Allowing this (as we currently do, see: test_update_signup_without_group_name_is_allowed) does conflict a bit with the test_nollning_event_signup_without_group test, since you can achieve a different end state by editing the signup.

[hannesblaman]

These changes will probably break signup update screen for non-nollning events. This will require small changes on the frontend but I need to test frontend changes some more, and since that probably won't be an issue until after nollning I will not prioritize this right now (next week maybe). This should work for now.

[georgelgeback]

Changed it so that we use exclue_unset=True which means any field not sent by the frontend will be left alone. Added some more tests to ensure this works (only checked priority because that required the least changes to tests and group name is already kinda checked afaik). We did need frontend changes to make clearing priority from signup card possible: fsek/WWW-Web#320

[georgelgeback]

LGTM