Use meetup time instead of start time for calendar export

[lukasrad02]

Is your feature request related to a problem? Please describe.
When using the calendar export, each calendar entry will use the start time of the event as start time of the calendar entry. There is no way to find out the meetup time from a calendar export and editing the calendar entries is not possible for imported ical calendars.

Describe the solution you'd like
Let each user select whether they want their calendar entries to start at the beginning or meetup time of their events. Additionally, include the other time somewhere in the calendar entry (e.g. in the title or description).

Describe alternatives you've considered
As an alternative (easier to implement but poorer UX), do not let each user choose, but still include the meetup time at some prominent spot of the calendar item (e.g. the title).

Additional context
This issue is based on the EvaP feedback for the Erstitage at HPI.