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'```
Because @ametaireau did here https://github.com/mozilla-services/loop-server/commit/f95250a12e4a400db574bdcd1f331022e4471754#diff-2fbbe66e501e7e82cd4c1868a4632212R258