Updating some section slugs for visas browse pages

Some content titles have changed to aid SEO. This changes the slugs to match the new titles.
Pivotal https://www.pivotaltracker.com/s/projects/1008986/stories/70510328
Merge pull request #181 from alphagov/update_some_ukvi_browse_slugs

Adding migration to update some section slugs for visas browse pages