EventReminderNotification uses confusing wording

[jeriox]

The reminder notification always says Your support is needed for {title} regardless of the event type or settings. While this is sensible for Sanitätsdienste and so on, it can be confusing for Zugabend or similar where there is no need for support, but people should be reminded to participate.

Two ideas from the top of my mind:

• Check if the event has missing spots, then render something like the old message, otherwise use a more generic wording
• Let the user customize the notification text
  • including the event description could also be a sensible option
  • we would probably need some kind of templating for that

related to #1041