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
Add optional content IDs to the store.

Once all the client apps are generating content IDs, we can make these
mandatory.

The UUID validation code is taken directly from the contacts app
<https://github.com/alphagov/hmrc-contacts/commit/98a099dee004>. It's
not worth the extra time right now to pull this out into a shared gem,
but it's something worth doing later if/when we want to use this in a
third place, or if we need to update the validation logic.
Test that we don't expose content IDs to the site.

The content ID is designed to be visible only to the publishing tools
and the content store itself, not to the site-facing (and eventually
public-facing) side of the content store.

Thankfully, we restrict the presentation of content items with the
`PUBLIC_ATTRIBUTES` attribute anyway, so this doesn't get presented by
default.
Add `content_id` fields to example JSON.

This isn't included in redirects or "gone" items yet, because we haven't
confirmed for sure whether they should have content IDs.
Merge pull request #47 from alphagov/content-ids

Add content IDs
Merge pull request #46 from alphagov/links

Rename the new version of tags to `links`
Correct the exchange name in RabbitMQ docs.
Merge pull request #48 from alphagov/update-rabbitmq-docs

Correct the exchange name in RabbitMQ docs.
Remove url-arbiter feature flag.

This is now to be enabled everywhere, so the flag is unnecessary.
Merge pull request #49 from alphagov/remove_url-arbiter_feature_flag

Remove url-arbiter feature flag.