Add flag to disallow participant leaderboard control

For the AutoML challenge we need to force participants to not have access to adding/removing from leaderboard.

## Tasks

- [x] Adds `disallow_leaderboard_modifying` flag for `competition.yaml`
- [x] Competition edit form has `disallow_leaderboard_modifying` checkbox
- [x] If auto submit to leaderboard enabled AND `disallow_leaderboard_modifying` are set, show "Disallow modifying leaderboard" in place of buttons, like before