Add categories to events

[HerrLevin]

To de-clutter the complexity of events & the event overview page, Events should be able to have categories. That way, one can filter for pride events, festivals, Special train rides, etc.

Additional context
We should statically define the events with an enum and the corresponding translations rather than database-managed. That way, we will always have translations w/ a PR rather than possible new categories, that are not translatable by us/weblate.