Link to the Org from a specialist document

With certain documents, users will generally be linked straight to that
document and want to know that it comes from that Org. This commit adds
a `DocumentPresenter#organisations` method which pulls them from the
content API response, a helper to render them as a links in a sentence
and updates the view to display them.

Ticket:
https://trello.com/c/B8KLPUa6/387-drug-and-device-alerts-and-dsu-documents-should-be-tagged-to-the-mhra
Merge pull request #61 from alphagov/link-to-org-from-document

Link to the Org from a specialist document