To reduce the side effects of this change, we introduce a new UnknownId
action type:
- SingleUnkIdAction
This can be returned from functions to signal that we need to do some
work to look for things. We add a `must_use` directive to this type
to ensure callers handle it.
Changelog-Fixed: Fix missing profiles when new accounts are added
Fixes: https://github.com/damus-io/notedeck/issues/356