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

- [ ] Adds `disallow_leaderboard_modifying` flag
- [ ] If auto submit to leaderboard enabled AND `disallow_leaderboard_modifying` are set, show "Automatically added to leaderboard" in place of buttons, like before