Rename the new version of tags to `links`

This is as a result of a few discussions about potential naming conventions, and the benefits and disadvantages of each. Most of the disadvantages stem from clashes with existing terms, and the potential for confusion.

In the world of [Panopticon](https://github.com/alphagov/panopticon) and the [content API](https://github.com/alphagov/govuk_content_api), tags are treated as first-class objects, with their own attributes and persistence; in the content store, we're using them to express a typed relationship between two pieces of content. This may conflict with existing mental models, both of tags as they exist in the GOV.UK publishing system, and of tags in the wider world as keywords designating a subset of objects.

Some possibilities we've considered:

  * `tags` – clashes with existing naming convention, and the existing concept of tags on the Web
  * `links` – clashes with the concept of hyperlinks, although our planned implementation maps to HTML links and the `rel` attribute <http://www.w3.org/TR/html5/links.html>
  * `related` – clashes with the fact that we're expecting `related` to be one of the link types we use
  * `associations` – on the plus side, no pre-existing mental model to clash with; on the minus side, no pre-existing mental model at all