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

For @jarv to look over. This has been looked at and tested by @jrbl and @jbau, and seems ok in nature.