Feature: Event Filter using Categories ie. Courses

[PhictionalOne]

Having added placeholder events recently, made the inclusion of all kinds of events possible. We used the feature for the course subject introductions, the semester opening and the Schnuppersitzung.

The latter were easy to include. For the course subject introductions we opted to bundle them up in one event though they were spread over multiple days and locations. We retained a simpler overview that way but it is unsatisfactory.

I propose we add a schema to the events.yaml:

SUBINTROLEHRAMT:
  # [...] 
  categories:
    - lehramt
    - ersti

SUBINTROKOGNI:
  # [...]
  categories:
    - kogni
    - ersti

# Exclusion example 
WORKSHOPLATEX:
  # [...] 
  categories:
  - except-ersti

Omitting categories should be the default and be interpreted as applicable to all categories. A stretch goal could be to have category exclusion. In the event we start to hold workshops again, we might want to not distract first semesters with a latex workshop meant for second semesters or the like.

The new category tag could then be used to...

• add category buttons to the UI beside the language selector, as well as an "All" button
• filter using the GET parameter ?category=lehramt omitting that can display should display all. (both in web view as well as ics/calender generation)
• show an visual indicator on the event cards