WIP: Use approximate comparison when checking for changes to start dates
Test start_date in course settings
Assert date equality directly, rather than by using a delta of 0
Remove extraneous test for already handled edge case
Make DraftModuleStore mongo only

DraftModuleStore was originally designed as a mixin, but never used that
way, and with the upcoming changes to use the versioned module store,
never will be. This changes removes a circular dependency between
mongo.py and draft.py.
Force locations to be Location objects earlier
Customtags can't be inserted by studio
Work around XBlock limitations to force persistance of field mutation