Feature/cancel initial subscriptions

Add the cancel_subscription workflow that allows for taking a subscription from INITIAL to TERMINATED.

Since it's a termination workflow, we can't change this from showing up differently (they're forcefully all called "Terminate router" in this case). But only one is available to run, so I'd say this is acceptable.

Also, I added some subtile warnings to prevent accidentally running this.

gso/migrations/versions/2024-03-21_734e36a3e70b_add_subscription_cancellation_workflow.py

@@ -10,7 +10,7 @@ from alembic import op
 
 # revision identifiers, used by Alembic.
 revision = '734e36a3e70b'
-down_revision = 'd61c0f92da1e'
+down_revision = 'a2cd3f2e6d7a'
 branch_labels = None
 depends_on = None