bug: deleting a string does not remove the record from the PDS #437

When deleting a string via the appview, strings.Strings.delete only removes the row from the local database via db.DeleteString. It does not call comatproto.RepoDeleteRecord to remove the sh.tangled.string record from the user's PDS.

By contrast, repo.Repo.DeleteRepo, state.State.Star, and other record types all call comatproto.RepoDeleteRecord before removing the local DB row.

This leaves orphaned records on the PDS that are no longer visible on the appview.