add distinct 'admin' role separate from 'editors'. Only 'admins' should be able to add/remove editors.

per feedback, use reverse() when generating callback urls. Also generate thumbnails when ingesting static images. We'll need these handy for the CMS UI.

Support asset library references in CapaModule. Also some code refactoring/cleanup. Preparing for review and submit

Make imports into the CMS ignore broken modules, while still loading bad xml as error modules in the LMS

Define 'system metadata' which should not be editable by the end user. When posting back metadata edits, we need to fetch a copy of the existing metadata and apply the changes.

Make saving also save the children in the #sortable element (TODO: name the #sortable element better)

* New format: {tag}://{org}/{course}/{category}/{name}[@{revision}]
* Updated tests, code
* Added test chapter with : in url_name in toy course
* added test html page with : in url_name
* added a note to docs

- now handling access control via get_module

* update templates and code references
* also a display_name property that defaults to cleaned url_name

* CMS no longer syncs, it just imports when it gets a commit note
  from github

Only import from github when changes are made, rather than exporting. This prevents bugs in the cms from preventing changes via git

* make logging work properly in cms, dev
* fix a comment

* so they get the staff view of problems
* does NOT actually set User.is_staff

* change references and tests
* add staff/non-staff display
* added is_staff to ModuleSystem

Re-add __init__.py for contentstore that was accidentally deleted in 05add581