It doesn't always create a new room.

So that when a room is closed, a previously obscured chat doesn't become
visible again. Allowing this would force us to know which chat is visible, so
that we can falsify it's `hidden` attribute.

otherwise also the prototype's defaults gets overwritten.

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.