Instances of `RegexError` are just asserted as `assert_not_reached` as
they cannot really fail except for allocation failure if the given regex
is valid.
For members-only, non-anonymous groups that do not have a name set we now
show an automatically generated name that is generated from the list of members
(offline, and online). Only the first name (before the first space) is used
to keep the generated name short.
The term *private room* is coined as a short hand for members only&non-anon group
chats.
This commit also uses the offline members list instead of the online member list
to generate avatar tiles (also only in members-only, non-anon groups.)
