Move edit info into xblock and runtime mixins
LMS-11183, LMS-11184
Implement edit info as mixin for split
LMS-11183, LMS-11184
Change published_date to published_on
to make consistent with other edit info timestamps
LMS-11184
Move tests into appropriate directory structure.
Implement edit info as mixin on base mongo
LMS-11183, LMS-11184
Merge pull request #5001 from edx/split/edit_info

Split/edit info
Merge pull request #5043 from edx/christina/cohort-selector

Move tests into appropriate directory structure.