validate nullness in the db on a bunch of columns

fixes CNVS-7414

Change-Id: I60978f66d88d916dc5736dbb975715172bd1e14b
Reviewed-on: https://gerrit.instructure.com/23141
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

Conflicts:
	app/models/learning_outcome.rb
	app/models/rubric.rb
	spec/models/assignment_spec.rb
wiki_page_comments doesn't exist anymore

Change-Id: Ic74c2a9ef33b03e68aa54b9fa25806175be528e8
Reviewed-on: https://gerrit.instructure.com/24336
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Tested-by: Cody Cutrer <cody@instructure.com>