Add telemetry for how many people use breadcrumbs

Fixes https://github.com/vector-im/riot-web/issues/9207

src/Analytics.js

@@ -84,6 +84,11 @@ const customVariables = {
         expl: _td('Whether or not you\'re using the Richtext mode of the Rich Text Editor'),
         example: 'off',
     },
+    'Breadcrumbs': {
+        id: 9,
+        expl: _td("Whether or not you're using the 'breadcrumbs' feature (avatars above the room list)"),
+        example: 'disabled',
+    },
     'Homeserver URL': {
         id: 7,
         expl: _td('Your homeserver\'s URL'),
@@ -234,6 +239,11 @@ class Analytics {
         this._setVisitVariable('RTE: Uses Richtext Mode', state ? 'on' : 'off');
     }
 
+    setBreadcrumbs(state) {
+        if (this.disabled) return;
+        this._setVisitVariable('Breadcrumbs', state ? 'enabled' : 'disabled');
+    }
+
     showDetailsModal() {
         let rows = [];
         if (window.Piwik) {