Rename industry_sector to specialist_sector in URLHelper

Change the URLHelper's edge-case-handling for industry_sector tags to
match on specialist_sector instead, as we're renaming the tag type.
Bump govuk_content_models to 7.0.0

This includes the renaming of industry_sector to specialist_sector in
the list of an artefact's tag types.
Merge pull request #155 from alphagov/change-industry-sector-to-specialist-sector

Rename industry_sector to specialist_sector

https://www.pivotaltracker.com/story/show/65625746