Update edition comparison view

This improves the diff view: now shows a more a detailed difference view
that highlights individual word changes.
Merge pull request #260 from alphagov/improve-diff-viewer

Update edition comparison view
Bump govuk_content_models

This new version does not include the saving of diffs on publishing.
Publisher now calculates diffs on demand.
Merge pull request #261 from alphagov/bump-govuk_content_models

Bump govuk_content_models