Track UISIs in bulk

Piwik supports sending an event value, which we can use to indicate
cardinality of UISIs to be tracked instead of tracking them individually.

This means we can track them at a lower frequency of (fairly arbitrary)
60s.

src/Analytics.js

@@ -199,9 +199,9 @@ class Analytics {
         this._paq.push(['trackPageView']);
     }
 
-    trackEvent(category, action, name) {
+    trackEvent(category, action, name, value) {
         if (this.disabled) return;
-        this._paq.push(['trackEvent', category, action, name]);
+        this._paq.push(['trackEvent', category, action, name, value]);
     }
 
     logout() {