Add config and redirects for digital strategy reports

This should redirect publications.cabinetoffice.gov.uk to its new home
on GOV.UK. These mappings were originally created manually from a
spreadsheet and redirected using hand-crafted Nginx.
Make trailing paths canonical
Add basic test for subdomain
Merge pull request #463 from alphagov/redirect-gov-digital-strategy-reports

Add config and redirects for digital strategy reports