- can't change the xqueue one because there's data that uses it
- we checked, and no one has sent out survey links that use the old sha1() format

doing this because we'll need the LMS to be able to send the anon student id to backend services e.g. for peer grading, and passing two different anon student ids to xmodule (one for xqueue, one for other uses) is just asking for confusion.

- architecturally slightly questionable: the preview ajax calls goes to an LMS view instead of an input type specific one.  This needs to be fixed during the grand capa re-org, but there isn't time to do it right now.
- also, I kind of like having a generic turn-a-formula-into-a-preview service available

- courseware index view now redirects to most recent chapter, or first
- simplify the view a bit

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

* also add a check for valid location in modx_dispatch

* 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

- only relevant in tests due to start dates
- still irritatingly intricate logic...

* if error is in xmodule_constructor(), catch and return an ErrorModule
* if error is somewhere else in get_module(), return None

* Update all uses.

- use get_instance, not get_item
- finish the id->location rename for modx_dispatch arg

* 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.