automate down migrations on downgrade

[aoldershaw]

we have the ability to downgrade Concourse by pinning to an older Concourse version and triggering bump-concourse-to-* - however, if there were any migrations that were run since that version, we won't automatically apply the down migrations upon downgrade.

we'll need some sort of mechanism to do this automatically - it could be a part of the terraform job that runs before the actual terraform step. since the migrations are disruptive, we'll need to stop all running ATCs prior to running the migrations