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".