[FIX] mail: font-family of notification counter in systray

On firefox, the font-family of the counter was font-awesome instead of Roboto
because the counter was inside the <i> tag on which font-awesome rules applied.
This rev. simply moves the counter out of that <i> tag.

addons/mail/static/src/js/systray.js

@@ -36,7 +36,7 @@ var NotificationTopButton = Widget.extend({
             return this._super();
         },
         update_counter: function (counter) {
-            this.$('.fa-comment').html(counter);
+            this.$('.o_notification_counter').html(counter);
         },
         on_click: function (event) {
             var self = this;

addons/mail/static/src/xml/systray.xml

@@ -11,7 +11,7 @@
     <t t-name="mail.chat.NotificationTopButton">
         <li t-att-title='_t("Notifications")'>
             <a href="#">
-                <i class="fa fa-comment"/> <t t-esc="widget.get('counter') || ''"/>
+                <i class="fa fa-comment"/> <span class="o_notification_counter"/>
             </a>
         </li>
     </t>