Update room tags behaviour to match spec more
Fix Riot's behaviour with room tags after my cleanup in
https://github.com/matrix-org/matrix-doc/pull/1457 . Although, reading
it again, it's not clear how you're supposed to tell the difference
between a reverse-dns tag name and a legacy freeform text tag
(contains a '.'?) - I've left it detecting these as freeform text
for now.
