Big access control refactor

* 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)