Fix behaviour of popstate in remote_search_filter.js

• Popstate was firing on page load in Chrome.  The popstate event handler now does nothing if called with no state.
• Popstate wasn't appending the xhr=true param.  Move this 'appendage' into getResultsFromRemote.
Merge pull request #1419 from alphagov/fix-announcement-filter-popstate

Stats announcements - Fix behaviour of popstate in remote_search_filter.js