create an admin interface

[damian0604]

This is more like a long-term thing, but it would be really cool to have an /admin endpoint or similar, with functionality like sending reminders to participants or get overall activity stats.

This would make ancillary scripts like https://github.com/ccs-amsterdam/3bij3/blob/master/example-scripts-and-helpers/Email_reminders.py superflous and make 3bij3 easier to use, also for less tech-y people