Update search templates

The CSS and templates for mainstream search have changed. Since we were using their styles, this is now broken. In order to keep this from happening I have copied over the styles & templates.
Point form action at correct URL
Remove unused search styles
Pass line-height to mixin removing need for !important
Use versioned image asset

Stops it changing without our knowledge
Merge pull request #38 from alphagov/fix-search

Update search templates