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/PostStreamScrubber.js

@@ -70,7 +70,7 @@ export default class PostStreamScrubber extends Component {
     const unreadCount = this.props.stream.discussion.unreadCount();
     const unreadPercent = Math.min(count - this.index, unreadCount) / count;
 
-    const viewing = app.translator.transChoice('core.forum.post_scrubber_viewing_text', count, {
+    const viewing = app.translator.transChoice('core.forum.post_scrubber.viewing_text', count, {
       index: <span className="Scrubber-index">{retain || formatNumber(this.visibleIndex())}</span>,
       count: <span className="Scrubber-count">{formatNumber(count)}</span>
     });
@@ -100,7 +100,7 @@ export default class PostStreamScrubber extends Component {
         <div className="Dropdown-menu dropdown-menu">
           <div className="Scrubber">
             <a className="Scrubber-first" onclick={this.goToFirst.bind(this)}>
-              {icon('angle-double-up')} {app.trans('core.forum.post_scrubber_original_post_link')}
+              {icon('angle-double-up')} {app.translator.trans('core.forum.post_scrubber.original_post_link')}
             </a>
 
             <div className="Scrubber-scrollbar">
@@ -115,12 +115,12 @@ export default class PostStreamScrubber extends Component {
               <div className="Scrubber-after"/>
 
               <div className="Scrubber-unread" config={styleUnread}>
-                {app.trans('core.forum.post_scrubber_unread_text', {count: unreadCount})}
+                {app.translator.trans('core.forum.post_scrubber.unread_text', {count: unreadCount})}
               </div>
             </div>
 
             <a className="Scrubber-last" onclick={this.goToLast.bind(this)}>
-              {icon('angle-double-down')} {app.trans('core.forum.post_scrubber_now_link')}
+              {icon('angle-double-down')} {app.translator.trans('core.forum.post_scrubber.now_link')}
             </a>
           </div>
         </div>