[BUG] Linked agent relator type is not translatable

[wgilling]

What steps does it take to reproduce the issue?
Make an islandora object and configure at least one linked agent on the item. Set the site up to use a secondary language and then select the other language and see that the relator type is not translatable.

• When does this issue occur?
  When viewing any item in another language that displays a linked agent.

• Which page does it occur on?
  On the secondary language version of any default view of items as well as the edit form (if the form config is set up to allow edit of that field in secondary language) -- and it could also appear in any teaser or search results display is configured to also display the field.

• What happens?
  Even though the relator type is able to have a translation provided for it, the actual label is never passed through the drupal t() function, so it cannot be translated until this PR is merged.

• To whom does it occur (anonymous visitor, editor, administrator)?
  All users who could see items with linked agent values when viewed in a different language.

• What did you expect to happen?
  For example, if the second language is French, I'd expect to see a French translation for the relator type such as "Artist", I'd expect to see "Artiste" instead.

Which version of Islandora are you using?
2.x-dev

Any related open or closed issues to this bug report?
no

Screenshots:
English version of some linked agents on an example content item:

French version of the same item:

[wgilling]

I will be making a PR to address this shortly -- stay tuned.