chore: simplify activity organiser indication

[SilasPeters]

Currently, every activity has a name like BaCo | Fun activity!, whilst BaCo is stored as part of the string. Afaik, we literally do not store the organiser whilst I think we should be able to determine the organiser of an activity and generate the displayed name to enforce the desired formatting (the comittee is sometimes forgotten).

Point of this issue is: whether we can do it now without manipulating the database, or later in a replacing solution, the desired behaviour is as follows:

• The organiser of the activity is stored concretely or can be determined.
• The name of the activity is prefixed by default by the organiser name, instead of manually entering the comittee as part of the actvity name.
• iCal activities support the organizer field, and we could supply this field easily now when generating icalendars.