refactor(main): user list label

Change user list label to use the public chat label since BigBlueButton
is not including it as an aria-label anymore. Although there is an
`Users list` being set as aria-label but I did not find it at the locales.

config/label/main.js

@@ -1,6 +1,6 @@
 module.exports = {
   panel: {
-    name: 'app.userList.label',
+    name: 'app.chat.titlePublic',
     open: 'app.navBar.toggleUserList.ariaLabel',
     close: 'app.navBar.toggleUserList.ariaLabel',
   },