Support externally hosted comprehensive themes

The old Stanford-style themes can be pulled in from an external Git repo; no such provision was previously available for comprehensive themes.

• Introduce two new configuration variables:

• EDXAPP_COMPREHENSIVE_THEME_SOURCE_REPO: Git repo for the comprehensive theme

• EDXAPP_COMPREHENSIVE_THEME_VERSION: Git branch, tag, or revision to check out from the repo

• Introduce a new task, "checkout comprehensive theme", doing the equivalent of the previously existing "checkout theme" task. Check out the theme repo to EDXAPP_COMPREHENSIVE_THEME_DIR. If
  EDXAPP_COMPREHENSIVE_THEME_SOURCE_REPO is unset, continue to assume that EDXAPP_COMPREHENSIVE_THEME_DIR already exists and is populated with a comprehensive theme.

• Rename the "checkout theme" task to "checkout Stanford-style theme".