Set filter according to PR/Change in URL

E.g. https://zuul-server/#333 will set filter to 333
Introduce related configuration option

And always pass the full change url in github status payloads
Allow the status_url to be used in a pipeline start-message.

Change-Id: I7b9eccb1d218d65d8c6b01e042e6cce4f8aec89a
diff --git a/etc/status/public_html/jquery.zuul.js b/etc/status/public_html/jquery.zuul.js
index d973948..5c69bd1 100644
--- a/etc/status/public_html/jquery.zuul.js
+++ b/etc/status/public_html/jquery.zuul.js
@@ -52,6 +52,10 @@
 
         var collapsed_exceptions = [];
         var current_filter = read_cookie('zuul_filter_string', '');
+        var change_set_in_url = window.location.href.split('#')[1];
+        if (change_set_in_url) {
+           current_filter = change_set_in_url;
+        }
         var $jq;
 
         var xhr,