Some update tests for EntityTypesController
Another update test for EntityTypesController
One more root system update test
Get test for EntityTypesController 'editable' attribute

Also, clean up other unrelated comments
Two more *TypesController tests

Also, filter out deliberately broken tests during development
Only test response.body for virtual attributes...

...like 'is_editable'. Test assigns() objects for others.
More new MeasurementTypesController tests, and...

...minor tweaks to existing ones
Factor common code from EntityTypesController tests

...and tweak let!() to let() in MeasurementTypesController tests
One more MeasurementTypesController test

Also, factor common code
All relevant RelationshipTypesController tests
created_by || root? check for EntityTypesController#update

...unbreaks two tests. Also, rename render_* numeric methods
Fix and re-break some faulty tests

...also, add created_by || root? check to MeasurementTypesController#update
and RelationshipTypesController#update
Disallow update if other systems have data

...unbreaks three tests
Stub out is_editable attribute in serializers
Implement proper is_editable attribute
Provide a System to EntityTypeSerializer test

...since the serializers now test the current system
Comment on & expand type update permission checks
Check update_attributes and render_error if needed

...also, standardize on render_error in a few more places
Shorten methods to satisfy rubocop
Remove extraneous comment