Rename acquire_lock to select_for_update, add docstring

LMS internal auth refactor and start dates

Xqueue callback acquires lock on StudentModule to avoid race condition

- now handling access control via get_module

* 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)

Wiki pages can now be viewed from the course URL and the course nav is shown. It doesn't follow the user yet.

started gradebook design; moved a few nav classes

Feature/cale/queue config

Fix askbot URLs in the theme