Handle "removed" repository (or not explicitly configured yet)

[tegioz]

There is one repository operation not supported yet: when a repository is removed from the configuration file (or when it was never in the configuration file but it exists on the service).

We have a few options to handle this:

• Reset all repository permissions and set its visibility to private, making it not accessible until it's added to the configuration (IIRC this is what Sheriff does). This causes situations like the one with the CLOWarden repo last week when we transferred it to the organization and we couldn't access it. Not a big deal though, just wanted to mention it. Issues assignments were lost and not recovered when we got access back.

• Ignore it, like if we didn't want it to be handled by CLOWarden unless it's added explicitly to the configuration file.

• We could also archive it, although this is probably a bit too aggressive and could cause other problems.

What do you think @jeefy?