put external link search results in grey box

so that they look ok in mobile search results
fix search tabs in mobile view

stops the tabs sliding underneath each other in narrow viewports
scope search styles to containing <section> element

to stop style bleeding out to the .search class also placed on the body
element
Add metadata and summary info more Inside Gov search results

- add 'corporate information' metadata tag and result summary to
corporate information results
- add 'document series' attribute to document series result
Merge pull request #357 from alphagov/search-minor-style-changes

Search minor style changes