Use the new GOV.UK logo on 410 and 404 pages

- This makes Bouncer consistent with the rest of GOV.UK.
- This using an image version of the GOV.UK crown and text to avoid
  having to include frontend templates with loads of files in Bouncer.
Remove the GIF version of the GOV.UK logo as it's unused
Resize the GOV.UK logo for consistency

With GOV.UK itself.
Don't hide GOV.UK logo on hover

This was used with a sprite to show the logo with underlined text.
Remove unused CSS
Merge pull request #94 from alphagov/use_new_govuk_logo

Use the new GOV.UK logo on 410 and 404 pages