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/PostStream.js

@@ -221,7 +221,7 @@ class PostStream extends Component {
             if (dt > 1000 * 60 * 60 * 24 * 4) {
               content = [
                 <div className="PostStream-timeGap">
-                  <span>{app.trans('core.forum.post_stream_time_lapsed_text', {period: moment.duration(dt).humanize()})}</span>
+                  <span>{app.translator.trans('core.forum.post_stream.time_lapsed_text', {period: moment.duration(dt).humanize()})}</span>
                 </div>,
                 content
               ];