[FIX] website_forum: ensure media modal opens on click button

Clicking the media button failed to open the media modal. The reason was
that initializing the media modal involved triggering an RPC to fetch
attachments but that resulted in a never-resolving promise because
`trigger_up('call_service')` was never caught. That is because
`website_forum` failed to pass itself as parent of `wysiwyg` when
instantiating the editor, thus breaking the chain that should have led
to `public_root`, which listens to `'call_service'` custom events.

closes odoo/odoo#62991

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/website_forum/static/src/js/website_forum.js

@@ -181,7 +181,7 @@ publicWidget.registry.websiteForum = publicWidget.Widget.extend({
                     </div>`,
             };
 
-            await wysiwygLoader.loadFromTextarea(this, $textarea, options);
+            await wysiwygLoader.loadFromTextarea(self, $textarea, options);
         });
 
         _.each(this.$('.o_wforum_bio_popover'), authorBox => {