Also... `num_unread` is now updated for groupchat messages in which the user is
mentioned.

and I'm not aware of a valid usecase for doing so.

Otherwise it gets rendered multiple times (at least during testing).

Removed the "fetching_messages" hack.

Otherwise they don't appear when reloading the page.

I'm not completely happy with the resulting workaround to avoid that messages
restored from sessionStorage increment the unread messages counter, but it'll
have to do for now.

Causes login failure when one-time tokens are used.

We need this because when the plugin listens to a promise, we can't trigger it
again later (after adding our spy), we instead need to add the spy beforehand.

by first waiting until chatboxes have been fetched as well.