FEATURE: don't notify about changed tags for a private message (PR #10408)

Only staff members observing specific tag should receive a notification


This line doesn’t seem to be used :stuck_out_tongue:

I think this test is a false positive because the newly fabricated user is not an allowed user of the PM. As a result, I don’t think the revision is successful. The API is confusing here because it is revise! with a ! which seems to indicate it would raise an error.