Add third tier to key namespacing

- Changes all `app.trans` calls to `app.translator.trans` calls.
- Changes existing keys to [three-tier namespace structure](https://github.com/flarum/english/pull/12).
- Extracts additional strings for `lib:` namespace.
- Extracts two previously missed strings for EditGroupModal.js.

js/forum/src/components/UsersSearchSource.js

@@ -22,7 +22,7 @@ export default class UsersSearchResults {
     if (!results.length) return '';
 
     return [
-      <li className="Dropdown-header">{app.trans('core.forum.search_users_heading')}</li>,
+      <li className="Dropdown-header">{app.translator.trans('core.forum.search.users_heading')}</li>,
       results.map(user => (
         <li className="UserSearchResult" data-index={'users' + user.id()}>
           <a href={app.route.user(user)} config={m.route}>