Name variants on item pages

[Atmire-Kristof]

This PR adds support for name variants on related items by always using the virtual metadata’s value.

This is achieved by modifying ItemMetadataRepresentation to accept a MetadataValue in the constructor and return its value in the getValue() method of the representation.
This getValue() method is then called on the entity-metadata-list-element components to display the name variant.

A sample item is https://dspace7-entities.atmire.com/items/72635f7f-37b5-4875-b4f2-5ff45d97a09b

[paulo-graca]

I'm experiencing the same as in #475, where the ds-metadata-representation-list component doesn't behave as expected using my local backend.

[paulo-graca]

But, as in #475 , this is working as expected using the Atmire backend. As an example, the Person with the UUID: 0ffbee3f-e7ea-42bc-92fe-2fbef1a52c0f has this metadata:

"person.familyName": [
      {
        "value": "Eens",
        "language": "",
        "authority": null,
        "confidence": -1,
        "place": 0
      }
    ],
    "person.givenName": [
      {
        "value": "Marcel",
        "language": "",
        "authority": null,
        "confidence": -1,
        "place": 0
      }
    ],

but the author name presented in the Publication with the UUID 72635f7f-37b5-4875-b4f2-5ff45d97a09b is "Eens, M." when, based on the default behavior ([person.familyName], [person.givenName] ) it should be: "Eens, Marcel".

[paulo-graca]

Why the line break?

[artlowel]

@Atmire-Kristof removed it

[paulo-graca]

Using Atmire's backend. This PR works as expected but I can't test it with my own backend. As I referred in #475 I think this might be related with some backend dependency I don't have in my local environment.

[benbosman]

I have quickly tested this with the latest master REST and this branch in Angular, and I noticed dc.contributor.author contained the name variant and the authors section on the simple item page contained the name variant as well, without any errors

[tdonohue]

Code looks fine to me. I haven't tested it, but trust the testing of others.