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/PostMeta.js

@@ -33,7 +33,7 @@ export default class PostMeta extends Component {
         </a>
 
         <div className="Dropdown-menu dropdown-menu">
-          <span className="PostMeta-number">{app.trans('core.forum.post_number_tooltip', {number: post.number()})}</span>{' '}
+          <span className="PostMeta-number">{app.translator.trans('core.forum.post.number_tooltip', {number: post.number()})}</span>{' '}
           {fullTime(time)}
           {touch
             ? <a className="Button PostMeta-permalink" href={permalink}>{permalink}</a>