"Attended" field for event signups

Created by: NotSpecial

In #82 the idea for a GV-Tool was mentioned.

I think this can be solved (and other cases) by a more general solution in the API:

An attended field for signups

This field should have three possibilities in my opionion: Null, False and True

• Null: Not relevant
• False: Did not show up
• Did show up

Important: This field can only be set by an admin (or maybe by the event owner as well?)

This could be used in several situations:

• Events with limited slots: It should be relatively easy to delevop a tool to get a checklist for event participants (can also be used to track payments)
• GV Tool: Such a event with tracked attendance could be used for the GV as well