Project index block view includes sandbox projects

Part of #1031578: [Meta] Reflect a consistent strategy for sandbox visibility was that sandbox projects weren't supposed to be visible by default. Project provides a "Module index" view, a block of which is visible at http://drupal.org/download ... the !&# Content project at the top of that list is a sandbox. #fail! ;)

Luckily, this should be an easy fix. Also related to #1075206: Feed for "New modules" includes sandbox projects → feed for new "full projects"? since we're probably going to want to add a feed to this view using the same filter...