Properly clean up after federated account deletion

[tsmethurst]

Right now we handle account deletion like this:

https://github.com/superseriousbusiness/gotosocial/blob/main/internal/federation/federatingdb/delete.go#L89-L102

Ie., we just straight up remove the account from the database.

But then we don't handle side effects like removing followers:

https://github.com/superseriousbusiness/gotosocial/blob/main/internal/processing/fromfederator.go#L182-L185

This leads to bugs where going through followers/following lists fails because accounts pointed to by follows are removed (see #251).

We should implement proper exhaustive behavior for deleting accounts, so that stubs of follows/statuses/likes/etc aren't retained in the database at all.

After that we can consider rolling back the quickfix of #251

[tsmethurst]

Closed by #254 but we might want to have another discussion about this at some point.