Oferko/proposed/mark related exs

These commits relate to the feature requested in the meeting last week to designate in the homepage which videos have exercises associated with them.
I'd be happy if you guys can review this. You can see it at work at [here](http://stars.hebkhan.appspot.com/).
Main consideration - performance. You can see [here](https://appengine.google.com/logs?app_id=s~hebkhan&version_id=stars.371709739947407753&severity_level_override=0&severity_level=3) errors in the appengine logs that seem to only occur the first time the homepage is loaded (before it's cached). Shay - maybe there's a way to avoid these issues?