Allow plugins to add panels to header more easily

app/assets/javascripts/discourse/widgets/header.js.es6

@@ -214,6 +214,8 @@ export default createWidget('header', {
       panels.push(this.attach('user-menu'));
     }
 
+    this.additionalPanels(attrs, state).map(function(panel) { panels.push(panel) })
+
     const contents = [ this.attach('home-logo', { minimized: !!attrs.topic }),
                        h('div.panel.clearfix', panels) ];
 
@@ -224,6 +226,11 @@ export default createWidget('header', {
     return h('div.wrap', h('div.contents.clearfix', contents));
   },
 
+  // override to allow plugins to append additional panels
+  additionalPanels(attrs, state) {
+    return []
+  },
+
   updateHighlight() {
     if (!this.state.searchVisible) {
       const service = this.register.lookup('search-service:main');