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'```
Yes, me too.

It is supposed to be here:
https://docs.services.mozilla.com/loop/apis.html#get-rooms-token
https://wiki.mozilla.org/Loop/Architecture/Rooms#GET_.2Frooms.2F.7Btoken.7D