Fixing sequentials nested inside of verticals, provides a path to sequentials nested in sequentials. Changes jump_to to redirect to courseware index.

* it now works in the context of a specific course_id
* add tracking of parent locations to xml modulestore
* adjust lots of tests, including some refactoring
* NOT working yet: jumping to the right position in a sequence.

* add a url that has an extra /{position} at the end
* pass it through to get_module

* All access checks now done for the "user" param, ignoring request.user
  - This matters for xqueue callbacks and for instructor view of student progress
* An effect of this change is that if a student couldn't see their own 'progress' tab,
  the course instructor won't be able to see it either

* currently only in the staff view of student progress page

[Fix #34379687]

* hopefully won't come up, but if they do, our 500 page
  should be better than  HttpResponse("There was an unrecoverable error")

* 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.

- don't call has_access directly from template, pass a staff_access variable instead

* error modules now hidden via access control
* get_module() returns None if user doesn't have access

* All access control logic is now in access.py
* It exports a single method for general use: has_access(user, object, action)
 - possible actions depend on object type (e.g. 'see_exists', 'enroll', 'staff')
* Removed DARK_LAUNCH feature flag--it is now the default behavior
* Replaced check_course with three separate more focused functions that use has_access

Minor things:
* note on using pdb in testing
* moved time parsing helper into timeparse.py
* x_modules now have a .start attribute (None if not in metadata)

* pass user to check_course
* if dark launch feature enabled, users with staff access to course
can see courseware before start date.  Students still can't.
* tests.
* Remaining: enrollment view has custom access control.  Need to check it.

* dashboard links to gradebook and dummy grade summary page

* also added todo about need for pagination

* we appear to be fairly consistent in using is_staff and has_staff_access_to_course
* cleaned up some docstrings and little code things as I went
* fixed small bug in change_enrollment view (check for non-anon user)

* use url_name to compare to passed params
* remove unused fields from template context
* Also make it possible to link to a chapter