Todo: Override persistence: Remove overrides from db for a channel when it is deleted

[FloatingMilkshake]

Would help avoid a build-up of overrides in the db for channels that no longer exist

[FloatingMilkshake]

This already exists, actually! But some overrides were left behind in the db; their channels were probably deleted before Channel Delete was handled.

I have however added a command debug overrides cleanup to retroactively clean up these overrides in 679f311