RDF UI: Map multiple ontologies to a field

[kimpham54]

Title (Goal)	RDF UI: Map multiple properties to a field
Primary Actor	Repository admin
Scope	access
Level	Low
Story	As a repository admin, I want to be able to select multiple properties (including from different ontologies) to map to the same field of a bundle resource/entity type. This use case depends on a previous use case #485 , the ability to load multiple ontologies.

[DiegoPino]

@kimpham54 just a quick update on this. I suppose this should state: "Map multiple properties coming from different ontologies to a field". Because the mapping that happens in Drupal 8 is field to property, not the whole ontology

[kimpham54]

@DiegoPino thanks, you're right! I made the fix

[dannylamb]

Transferring to Islandora-Labs/rdfux#3