[Delivers #51842627] Linker with multiple items

This solution precludes any amendment to the data model by allow aggregation of linker records upon creation. 

When creating a new subject/agent link, allow linking to multiple records via the linker widget e.g. typeahead will allow multiple items and the Browse popup will offer checkboxes to select multiple items.

Upon save, these items are parsed to create the distinct link records.  The resultant form will show a separate link record for each of the items in the linker - the linker widget will now return to one-to-one mode and offer radios and a single record only.