Improve links on "Culture that supports change"

- More accurate link text
- Bring out GOV.UK changes into its own section
- Add details on the recent hosting migration
Link to a fragment in privacy note example

It's nicer than linking to a page that doesn't exist.
Fix broken links

- Valid mailto URIs don't have a space between the protocol and address
- Several 'Digital by Default' links don't work
- 'writing-government-services' was deleted in 11aab6dd5ec26a370511b80a652e9e66acab60d0
  but some links still remain (see also bc20a52c1bc7b888fffaa7cb2dfda8c3d62f5106)
- Broken CloudStore link
Tweak Makefile

- Correct _site directory name
- Add it to exclude list so it isn't compiled as part of the site
Add link checker tool

This is super hacky and isn't anywhere near mature enough to be part
of the build process, but it's highlighted some important problems
so I consider it worth including.

Run it with ./tools/links.rb after building the site locally. It will
test GOV.UK production for relative links.

There's a fairly generous pause between each HTTP request.
Follow redirects on external hostnames

We shouldn't assume that every website will be as good as we try to
be when changing URLs. While they redirect now, they could stop
working at any time, so we should update the service manual to
reflect the canonical location.
Switch some domains to TLS rather than plain HTTP

- en.wikipedia.org
- bing.com
- groups.google.com
- stackoverflow.com
- www.cesg.gov.uk
- www.gnupg.org
- www.vagrantup.com
- www.ietf.org
- www.youtube.com
- research.google.com
- www.nationalarchives.gov.uk
- www.muji.eu
- www.adobe.com
- www.surveymonkey.com
- econsultancy.com
Merge pull request #344 from alphagov/url-tweaks-and-improvements

URL tweaks and miscellaneous improvements