Fix teams structure fullness display

[felixrindt]

Fixes #1381. Fixes #1382.

• add discussion points to Signup refactoring follow-ups #1383

This is just a quick fix. We're unhappy with the inconsistencies in the assignment, fullness-determination and counts between the signup structures and therefore plan to follow up with a design session (considering all possible combinations and scenarios and what behavior we expect of them), a rewrite of much of the logic touched here and user documentation.

(There is for example another bug affecting the "full"-state for atomic blocks in the complex structure, which can't be fixed without a more thoughtful rewrite of the matching algorithms.)

@lukasrad02 feel free to test this. As we discussed in our call, this should fix the issues you're currently having with your events.

[coveralls]

coverage: 85.283% (-0.04%) from 85.323%
when pulling df80fb6 on fix-teams-fullness-display
into d719292 on main.

[lukasrad02]

I've noticed that there is still an inconsistency in the displayed number of missing people. Is that intended?

Scenario:

• Direct Confirmation, two named teams, each min 1, max 1, no required qualifications
• Participant "Admin" signed up for team A explicitly
• Participant "User" signed up without choosing a team
• The shift view shows 2/2 participants in total (and 0/2 for the second shift)
• The event list shows 3 missing participants, although it should only be 2