In members-only private MUC rooms, show offline users in the sidebar

Forked from #12417 (comment 219352):

[…] for members-only-rooms its useful to also see offline members in the sidebar […]

I think that would make a lot of sense, members-only rooms are usually more "coworker teams" kind of scenarios (and those rooms tend to have less people in them than public rooms anyway), in which case it is nice to know at a glance who is and isn't in the room, when you are dealing with a bunch of teams, departments or projects...

Furthermore, from that same comment, but related to #12210 (closed):

when we gain the ability to show offline users in the sidebar, the count will still be correct as it always counts whats in the sidebar.