Search filters put into left column for tablet and desktop views

- added divs to allow column floats
- CSS for columns
- CSS copied from Whitehall for styling
- image asset copied from Whitehall
Wording changes to search filter

worked with @AmyWhitney
fix scrolling behaviour on search filter

- when the department filter is used on search pages, the page no
longer jumps back to the top
- JS change by @edds
- dependancy on
https://github.com/alphagov/static/commit/5438b01d529e625118b19461f55d93
30becee683
Patch styling bug with search filter in non-js desktop view
Search: additional whitespace above filter submit button
Ignore organisation filtering for top results.
Avoid duplicated top results in filtered searches.

Because we were using a separate, unfiltered list to extract top results from
filtered searches, the code to remove the duplicates wasn't working because it
compared results by identity. Determining result equality by whether they have
the same link solves this.
Merge pull request #355 from alphagov/search-filter-fixes

Search filter fixes
Merge pull request #356 from alphagov/unfiltered-top-results

Unfiltered top results