Fix broken URLs

This commit:

- moves GDS URLs over their redirects to the appropriate place
- fixes the link on the ideal alpha guidance
- changes some external hostnames to HTTPS

This reveals that some URLs are broken. We should fix them:

- http://www.cabinetoffice.gov.uk/sites/default/files/resources/greening-government-ict-strategy.pdf redirects to a publication that does not exist
- http://www.publications.cabinetoffice.gov.uk/digital/strategy/ doesn't work
- http://www.publications.cabinetoffice.gov.uk/digital/strategy/case-studies/government-as-platform/ doesn't work