Enforce type of the XBlocks Fields when set

This updates the XBlock dependency to edX/XBlock@***
and add the newly introduced enforce_type methods
Add xmodule fields enforce_type tests
Make enforce_type an optional flag, to avoid conflicts
enforce-type: set the edx/XBlock#200 merge hash
Add my name to AUTHORS
Allow turning off Video CDN in Studio.

BLD-1207
Annotator Tools: Fix major logic bug in code
Add Optimizely widget
Merge pull request #4760 from edx/oleg/turn-off-cdn-rc

Allow turning off Video CDN in Studio.
Merge pull request #4764 from edx/diana/show-cert-without-end-date

Show Certificates Before End
Merge pull request #4763 from edx/lms/add-optimizely

Lms/add optimizely
Merge pull request #4762 from edx/annotations-js-fix

Annotator Tools: Fix major logic bug in code (RC)
ORA2 events
Remove banner from instructor dashboard
cleanup
wrapping
Reenable user partition in Advanced Settings.
Merge pull request #4775 from edx/alex/fix-reenable-user-partitions

Reenable user partition in Advanced Settings.
Added Will's nuance and info on availability from Lyla
Use UrlResetMixin to test GroupConfiguration feature