Add instances to accessions (schema, backend and frontend)
Add 'accession' instance_type and allow modification of the instance type after adding the sub record
Offer 'Add Instance' and 'Add Digital Object' buttons instead of just the Add Instance dropdown button/with dropdown instance type selector;
Merge pull request #367 from hudmol/43799411-instances-on-accessions

[Delivers #43799411] Add instances to accessions