[FIX] mass_mailing: prevent dropping snippets outside mailing

The fix at [1] had to be adapted anyway since the conversion to OWL. As
a result it was possible to drop snippets below the mailing area.

[1]: https://github.com/odoo/odoo/pull/77724/commits/65b67d66de72fab390b8887c12532b840d47c95f

task-3002441

closes odoo/odoo#102972

X-original-commit: fe4006997daeb5d323b364821fc2815a45e78495
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/static/src/js/mass_mailing_html_field.js

@@ -136,7 +136,7 @@ export class MassMailingHtmlField extends HtmlField {
         // named so they are handled properly by the snippets menu.
         this.wysiwyg.$iframeBody.find('.o_layout').addBack().data('name', 'Mailing');
         // We don't want to drop snippets directly within the wysiwyg.
-        this.wysiwyg.$iframeBody.removeClass('o_editable');
+        this.wysiwyg.$iframeBody.find('.odoo-editor-editable').removeClass('o_editable');
 
         initializeDesignTabCss(this.wysiwyg.getEditable());
         this.wysiwyg.getEditable().find('img').attr('loading', '');