Add feature: Ability to mark a team as DNF (did not finish) to provide better visibility

[ometa]

Add ability for a checkpoint volunteer to mark a team as dropping out of the race, and have that information persist through the end, and be visible to other checkpoints and to the admin panel. This give better visibility into the status of the race, and allow to shut down early if desired.

Implementation details

Similar to allowing each checkpoint to mark a team as bad (customizable, per checkpoint tags), this feature needs to allow each checkpoint to add a new status, similar to "bad", but DNF. It would not prevent the team from being checked in. It's more about adding another label to a team. We do need to expose the label to all other checkpoints vs. gating it behind the admin.