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.
Well, each parameter can occur multiple times, so we'd need to handle values which are arrays for this.  An array of arrays seemed simpler to me.  I suppose we could automatically add the [] for values which are arrays, which might be nicer - I'll give that a go and see how that looks.