Merge branch 'merge-polling-with-master' of https://github.com/bigbluebutton/bigbluebutton into merge-polling-with-master

Reverse the check for http response code.

There are many codes other than '404' which indicate that the media
file is not present or otherwise unusable. Instead of checking for
a particular failure mode, check for explicit success.

I ran into this on a server that had directory listing disabled, and
returned a 403 (access denied) error on non-existant files.

 - fix url for slides

 - the whiteboard id matches presId/page id for the current page