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/admin/src/components/SessionDropdown.js

@@ -43,7 +43,7 @@ export default class SessionDropdown extends Dropdown {
     items.add('logOut',
       Button.component({
         icon: 'sign-out',
-        children: app.trans('core.admin.header_log_out_button'),
+        children: app.translator.trans('core.admin.header.log_out_button'),
         onclick: app.session.logout.bind(app.session)
       }),
       -100