Group related links based on section

This builds on the changes to the content api (see alphagov/govuk_content_api#119) to group all internal related items by their `group` value. Related items will now appear in one of three lists - either related items in the current subsection, in child subsections, or from elsewhere on GOV.UK.

There is also a workaround in here for travel advice artefacts, which removes the stub 'Foreign travel advice' section - set by Frontend for breadcrumbs - from the section titles in the related links list.