Validation: cannot rename criteria post-release
Merge pull request #363 from edx/will/TIM-574-take-2

Validation: cannot rename criteria post-release
Fix developer docs
Test datetime fixups
Merge pull request #371 from edx/will/fix-dev-docs

Fix developer docs
Merge branch 'master' into ai-grading

Conflicts:
	apps/openassessment/xblock/test/data/invalid_rubrics.json
	apps/openassessment/xblock/test/data/valid_rubrics.json
	apps/openassessment/xblock/test/test_message.py