Add a visually hidden result count

For aria-live to work, the bit that is read out needs to remain on the page (not be swapped out by the javascript). Putting a visually hidden result count that remains on the page and is then updated by the javascript means that when the result set has finished updating screen readers will read out the new result count.
Extract result_count_string to method

So that we can call it from the @results object outside of mustache for the aria-live result count, extract this to a method.
Add JS to update div outside of mustache

When the results have been loaded, JS now updates an aria-live region on the page with the result count. This means screen readers will read out the new result count.
Add a couple of tests for new aria updating
Add aria-hidden to mustache count

For screen readers the count appears outside of the mustache template so it is always on screen and always read out, so we can aria-hidden=true it here so it doesn't get read twice.
Merge pull request #656 from alphagov/improve-aria-roles-for-live-search

Improve aria roles for live search