Notifications generated for topics one may not access

If someone mentions a non-staff member, say @jane_doe, in a staff-only chat — then a notification about the mention is sent to that person (i.e. to Jane Doe). Then, when s/he clicks the notification, s/he gets an access denied error. (Thanks Burak for reporting this.)

Instead, no notification should be generated, and the people in the staff chat, should be informed about that somehow. (It can be useful to mention people in the staff chat, although they aren't notified — it's a quick way to link to their profile.)

A similar error happens if a non-staff member gets added to a staff-only chat, and then that person clicks the Join Chat button. Probably shouldn't be possible to add non-staff members to the staff-only chat, in the first place.