When you mention things in a Conpherence thread, the objects get a transaction which looks like "epriestley mentioned this in Restricted Conpherence Thread" to everyone who can't see the original mention.
At a minimum, we should stop these from showing up, e.g. by blacklisting these edits as mention-generating (something like `$editor->setGenerateMentions(false)`?).
Possibly we should hide these transactions for users if they can't see the mentioning object. However, if we do that, I believe it will change the anchors on the page. Currently, each transaction gets a numeric anchor, like `T123#1`, `T123#2`, etc. If `T123#7` is a comment, but you see fewer mentions than I do, it might be `T123#5` for you.
We could fix this by switching to transaction IDs instead of sequential IDs (`T123#2983798`), or by numbering only comments (but then we can never hide them) or by numbering hidden transactions and just not showing them.