Fix signup issues

[georgelgeback]

This PR changes the following:

• Block a signup with default priority if a user has a higher priority which is used by the event
• Handing out spots to users with the automatic route doesn't break anymore if prioritized users > max event users
• Signups with null as priority are cast to default priority, which means people can be blocked by the rule above even when sending null or no field. I checked frontend and backend, and this should be fine.

Changes to the actual code is +22 -7, the rest are tests for regression.

[hannesblaman]

LGTM