[Delivers #41509633] Instance digital object links

A few things:

- tweaked the test factory#sample to allow exclusion of specific enum values (as to reject digital_object from the instance_type enum for general instance factory)
- add digital object relationship to Instances
- tweak associations mixin to allow singular mapping
- refactor instance frontend forms to use the template system.  Javascript switches form based on the selected Instance Type
- modified the subrecord_form partial to allow definition of a custom add action