Add image for the new (high res) GOV.UK logo

This logo has a higher resolution than the logo that's currently
available and includes the underlined hover state.
suggested links use c14n to match
quieten php warnings in tests
Set the new high-res GOV.UK logo for all 4XX pages

Added a slight fix for oldIE because of how it deals with margins.
Use Helvetica Neue as the default font stack

After some discussion with a few designers it was decided that we
shouldn't send the font across for the redirector error pages. It
would increase the payload for the pages by ~300kb for a single load.

Setting the weight to 400 for better legibility.
Remunge all the things
Remunged files, with manual removal of 'fabricatedurl' and 'placeholderunique'
Remunged Wales Office file
Remunge all the things
Remunge all the things
Merge pull request #161 from alphagov/use-high-res-logo

Use high-res logo and simpler font stack
Merge pull request #162 from rjc123/remungeallthethings

Remunge of all the files