prepopulating contact form link field with referer

if the referer is a gov.uk URL, then the "A specific page" field
for "What's it to do with" is prepopulated with the referring URL.
This saves the user from having to type the URL out.
making the gov.uk identification more robust
Merge pull request #30 from alphagov/prepopulating_link_with_govuk_referer

prepopulating contact form link field with referer