Setlist categories #120

Is your feature request related to a problem? Please describe.
There is currently a simple long list of setlists and no way to group them.

Describe the solution you'd like

• There should be a new widget in settings for admins allowing to configure setlist categories.
• Those categories should show when creating or editing a setlist. Choosing a category is optional. Only one category can be selected.
• Plus a corresponding filter field for the list of setlists.

Describe alternatives you've considered
None.

Additional context
The filter fields for songs and setlist lists should be shown inline for larger viewports and only for mobile in a dropdown.