Add cascade delete and fix db nullability
Add cascade delete and fix db nullability
Merge remote-tracking branch 'origin/feature/cascade_delete' into feature/cascade_delete

Conflicts:
	src/ggrc/models/directive.py
	src/ggrc/models/program_directive.py
	src/tests/ggrc/models/factories.py
	src/tests/ggrc/models/test_cascade_delete.py
Add cascading delete behavior to remaining models
Add unique constraint to control_assessment
Add cascade delete and fix db nullability
Add cascade delete and fix db nullability
Add cascading delete behavior to remaining models
Add unique constraint to control_assessment
Merge remote-tracking branch 'origin/feature/cascade_delete' into feature/cascade_delete
Remove remaining references to Transaction model
Allow nulls for control.directive_id
Fix behave tests for cascading deletes and NOT NULL columns

* add `_publish_attrs` for RiskRiskyAttribute
* remove `Transaction` cases for behave tests
* add FactoryStubMarker to fill in required linked resources
* change post_example(...) to recursively POST to make valid links
Fulltext search refinements

* enable empty query `q=&...` to return all visible objects (or counts
  therof)
* include `NULL` in list of visible contexts
* enable `counts_only=true` query parameter to return the count of
  matches per type
* deduplicate `results.entries`
CSS update for controlobjective icon set
CO icon added to styleguide
Merge pull request #80 from reciprocity/feature/control-objective-icons

Feature/control objective icons
Update AppEngine SDK 1.7.6 -> 1.8.1
Refined show/hide of "Map to this object" link in sidenav extended info
Story 52269321: Mapping from lefthand nav objects to page object