Support hashes for tags in content API helpers.

Instead of having to pass in slugs for some of the content API
helper methods, being able to pass in hashes means we can change
more fields in the stubbed responses.

Also refactor and tidy the tag response generation code to
remove a lot of the duplication.
Merge pull request #52 from alphagov/support-hashes-in-content-api-helpers

Support hashes for tags in content API helpers