Manually end a competition

[Sch3lp]

This is actually solves the malicious competitor issue in a different way. In a way we don't need security, or the distinction between an admin and a regular participant anymore.

The idea is that modifications to the competitors of a competition are blocked after a competition has been started.

• Ending a competition toggles started back to false

[Sch3lp]

Might not be necessary when we implement UNDO of start competition.

Ending also doesn't make a lot of sense, maybe archiving would.