Redesign homepage to new design

Updated the frontend toolkit gem to get the new mainstream brand colour
variable.

Hid the search in the top bar as there is a search just below in the
main content area.
Remove unused images
Remove campagin notification test

There are no campaign blocks on the frontpage. As such this test is no
longer needed. @KushalP will make a remove the code from slimmer which
looked for the promo slot.
Typo
Pointer for submit and remove mobile top search

Also minor tweeks to margins for consistency
Remove alt text, image link and IE7 fix

- @tombye and I decided the alt text isn't needed and doesn't add
  anything.
- The link exists below with better text. It doesn't also need to be
  around the image.
- IE7 was removing some of the black background due to the -ive margin.
  Remove it in IE.
Add widths for IE
Fix for IE6/7 percentage width rounding bug
Update top box colour to blue

- Update Gemfile for latest - new mainstream colour
- Add custom blue for frontpage top box
Merge pull request #395 from alphagov/new-homepage

New homepage