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'```
I think we should be explicit there as well:

    {
        roomToken: req.roomStorageData.roomToken,
        roomName: req.roomStorageData.roomName,
        roomOwner: req.roomStorageData.roomOwner,
        roomUrl: roomsConf.webAppUrl.replace('{token}', roomToken),
        maxSize:,
        clientMaxSize:
        expiresAt:
        participants:
        ctime:
    }