Bug 1103331 — Get room public information if not participating

Interpreted https://bugzilla.mozilla.org/show_bug.cgi?id=1103331 as I could.

I think it differs from what was proposed by @ametaireau, since I don't use the POST with ```action == 'join'```
Nevermind it is explicit here: https://github.com/mozilla-services/loop-server/blob/1103331-get-room-public-info-if-anonymous/loop/routes/rooms.js#L175-L186