Add endpoints to delete users, groups and participations from admin interface

1 job for admin-delete-stuff in 2 minutes and 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #2098
test

00:02:03

 
Name Stage Failure
failed
test Test
error: could not compile `medal`.

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: aborting due to 9 previous errors; 1 warning emitted

error: build failed
make: *** [Makefile:23: clippy] Error 101
ERROR: Job failed: exit code 1