Stats announcements - Fix behaviour of popstate in remote_search_filter.js

This fixes a couple of issues in the Statistics announcement filter script, one of which was causing a cache confusion between xhr and non-xhr versions of the page :-(

• 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.