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/TerminalPost.js

@@ -21,7 +21,7 @@ export default class TerminalPost extends Component {
     return (
       <span>
         {lastPost ? icon('reply') : ''}{' '}
-        {app.trans('core.forum.discussion_list_' + (lastPost ? 'replied' : 'started') + '_text', {
+        {app.translator.trans('core.forum.discussion_list.' + (lastPost ? 'replied' : 'started') + '_text', {
           user,
           ago: humanTime(time)
         })}