Draft: Implement basic event attendance indication

Lars Kruserequested to merge
feature/374-event-attendance into master

Everyone with the permission to view an event is now able to indicate particaption.

The following useful additions are missing:

  • use DELETE instead of GET requests (not possible with simple forms) via javascript for removing attendance
  • beautify the visualization of attendee count and submission button
  • implement a personal attendance calendar export

See #374

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