Changed create_enrollment to create_or_update_enrollment
all CourseEnrollment modifications route through create_or_update
purchased_callback now uses the change_mode method
Response to CR
response to CR
Teach OEE to consider old task_states when trying to recover from an xml mismatch
Centralize to a single GradingServiceError, so that it's caught when expected
Small fix
Assign a score of 0 to tasks with no attempts
Use correct set of OpenEndedChild states
Merge pull request #1686 from cpennington/hotfix-max-empty-sequence

Assign a score of 0 to tasks with no attempts
Fixed password reset message

LMS-1507
Adding a YAML file to demonstrate how to include anonymized user IDs in Studio elements.
Basic test fix
298.15 K is 25 C, not 0 (0 is defined to be at 273.15)

Signed-off-by:  <manuel.freire@fdi.ucm.es>
 BLD-410: Add tests.
Merge pull request #1656 from edx/anton/fix-acceptance-test

Video player: fix acceptance test.
Fix i18n:test rake task
Provide language codes for i18n testing
Merge pull request #1634 from edx/flowerhack/feature/refactor-to-track-enrollment

Flowerhack/feature/refactor to track enrollment