Rough cut of searchability and data model relationships related to custom attributes.
Intermediate work on linking customattributables and customattributevalues.
Merge branch 'epics/custom-attributes' into feature/custom-attribute-search

Conflicts:
	src/ggrc/assets/javascripts/models/custom_attributes.js

Also, correct relation between CustomAttributable and CustomAttributeValues.
Merge pull request #2062 from reciprocity/feature/custom-attribute-search

Rough cut of searchability and data model relationships related to custo...