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...
Add custom attributes to all the modals
improved elements stacking in modal
Added spinner and fix indentation
Add mandatory field star (no actual validation yet)
Send custom_attributes values to the server