[Meta] Rework structure level filter in contribution list

[Elblinator]

Currently:

1. If the structure levels are not connected with the countdowns, then a contribution displays every structure level from a participant.
   In the filters these structure levels are counted in 'No structure level:
   

2. The structure level filter option displays every available structure level even if a structure level is not yet represented on the list

Steps to see behaviour:

1. Add a participant
2. create three structure levels and assign two of them to the user
3. (if you have the default settings you do not need to do anything, if your structure levels are connected via countdown, disconnect them)
4. let your user speak on a LoS
5. navigate to participants > contributions
6. your user should be displayed with two structure levels
7. the filter should display three structure levels

Wanted:

• 1. It is correct that every structure level is displayed in the contribution list. However, it should be possible to filter for the singular structure levels. Example from the screenshot: Sam Smith (C, A, B) should be displayed if you filter for structure level A, B or C
• 2. The filter should only display structure levels which are represented in the contribution list (and 'No structure level')

[Elblinator]

new behaviour (02.04.2024): currently the structure levels are displayed if they are not connected with the countdowns and they are not filterable