Fixes: AN-657

[LMS-179]
[LMS-209]
[LMS-1345]

XModules continue to use their own interface for xml, but provide an
adaptor that makes the the XBlock interface available.

[LMS-179]

For seeing overview of system status, for deleting and loading
courses, for seeing log of git imports of courseware.  Includes command
for importing course XML from git repositories.

Added a lot of tests for additional coverage with some minor fixes
those tests discovered

Move mitxmako initialization to a startup module, called by lms.startup and cms.startup for both management commands and when run via wsgi

[DEVPAIN-7]

* has to be eager

* Update all uses.

* apply policy per-course, even if multiple courses share course and org fields
* keep descriptors separate in xml store, so that if two such courses
  change the same module in different ways, it works.  Such edits will need to
  merged on CMS import...
* add get_instance(course_id, location) method to replace get_item(location).
  Update all the call sites
* tests, including a 2nd toy course with same course and org.

* if there is a policy.json in the course dir, read it
* file format is a dict with keys {category}/{url_name}, and values metadata dictionaries
* apply the policy, overwriting keys that are in the xml
* then do metadata inheritance, inheriting any overwritten keys.

* also a management cmd to generate a policy.json from a course dir.