cdodge/add-progress-publish-api-endpoint: expose a new xblock runtime publish special cased event type 'progress' which adds an entry into the CourseCompletions table

add default setting for feature flag in common.py
Merge pull request #121 from edx-solutions/cdodge/add-progress-publish-api-endpoint

cdodge/add-progress-publish-api-endpoint: expose a new xblock runtime pu...