Parameterizing edx git repository location

* Creates global variable $platform_code_dir to store
  /opt/wwc/edx-platform, to replace the semihardcoded /opt/wwc/mitx that
  we had before.
* Plugs $platform_code_dir in a lot of different places
* Templatize sef's update script hack

This is the other one, rebased. I wanted to put it in a separate branch so there'd be no confusion during review about where changes originated from.  Replaces #72.  @jarv, @jbau?