Add a Search-Parameters header

This allows apps to add extra parameters, which will be submitted along
with any search entry made from the page.  They get transformed into
hidden input elements in the search form.

This is part of https://www.pivotaltracker.com/story/show/77617490 -
there'll be a follow-up PR on whitehall which emits a header value to
filter search results by organisation for some pages.