[Delivers #43799411] Add instances to accessions

The main change is to add the instance record to accessions and a new Instance Type "Accession".

Another enhancement was to allow editing of the instance type (previously a hidden input) - this is to support the use case where an instance for a accession, typed as "Accession", is cloned after a spawn to the resource -- instead of filtering this instance we allow the user to modify it.

I've also removed the dropdown for the "Add Instance" and now offer "Add Instance" (analog instances with form offering the instance type dropdown) and "Add Digital Object" (a digital_object typed instance).

https://www.pivotaltracker.com/story/show/43799411