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/HeaderSecondary.js

@@ -64,7 +64,7 @@ export default class HeaderSecondary extends Component {
       if (app.forum.attribute('allowSignUp')) {
         items.add('signUp',
           Button.component({
-            children: app.trans('core.forum.header_sign_up_link'),
+            children: app.translator.trans('core.forum.header.sign_up_link'),
             className: 'Button Button--link',
             onclick: () => app.modal.show(new SignUpModal())
           }), 10
@@ -73,7 +73,7 @@ export default class HeaderSecondary extends Component {
 
       items.add('logIn',
         Button.component({
-          children: app.trans('core.forum.header_log_in_link'),
+          children: app.translator.trans('core.forum.header.log_in_link'),
           className: 'Button Button--link',
           onclick: () => app.modal.show(new LogInModal())
         }), 0