Accepting/rejecting group membership request should mark notification read for all group admins

[boonebgorges]

1. Three users: A, B, and C.
2. A and B are admins of the private group Foo
3. C requests membership to Foo
4. A gets the email first, and visits the group membership page to accept/reject the request. When that page loads, A's corresponding notification is marked as read.
5. B's notification remains unread, even after A accepts/rejects.

This seems like a bug, right? The one concern would be if B shows up later to confirm the request, the notification will be missing. But I think it's clear enough in this case that the request has been handled by another admin.

Patch attached.

[dcavins]

I agree that the behavior you're suggesting makes more sense than leaving confusing notifications that can't be acted upon. Your patch looks good to me (passing the null value for user ID is clever and kind of surprising). Thanks for catching this weird behavior.

[boonebgorges]

In 11278:

When a group membership request is approved or rejected, mark all related notifications as read.

Fixes #7375.