Idea: add a command to aid in dropping out teams

[PeterJCLaw]

Currently this is manual and requires modifying several files. It would be great to automate these steps.

The flow is roughly:

• check whether the TLA exists
• check whether the team has appeared in any matches which have already been scored
• if the team hasn't appeared in any scored matches, decided whether to remove them from the compstate entirely
• if removing them entirely, remove from teams.yaml & layout.yaml and replace with nulls in league.yaml
• otherwise mark them as dropped_out_after the most recently scored match (maybe offer a mechanism to specify this as an argument in case a history value is needed, though the incremental value of doing so is pretty low)

It might also be useful to then prompt the user for whether they want to also commit the change, though it is not typical for CLI commands to do so. If adding this we might want to also check that the compstate is clean before starting.