notifications: untangle notification access check

This removes a broken permission check when viewing notifications (the
HasRepoPermissionAny object was created, but never actually called with
a repo_name argument as required). It would be non-trivial to actually
implement the check, as notifications don't track their repository
relationship explicitly, and even then, it's unclear why it would
make sense to allow a repository admin to see notifications to
other users.

It was never a vulnerability, due to a subsequent (and much stricter)
ownership check, which remains but has been untangled for readability.
In short, this changeset is a pure refactoring, except that specifying
a non-existent notification ID will now produce error 404, not 403.