[FIX] website_mass_mailing: restore basic edition of mailing popups

Following the new editor at https://github.com/odoo/odoo/pull/29775

,
the html field to edit the mass mailing popups in the backend appeared
entirely broken.

This was because the editor css was not loaded for that field.

closes odoo/odoo#34250

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website_mass_mailing/views/snippets_templates.xml

@@ -3,6 +3,8 @@
 <template id="iframe_css_assets_edit" name="CSS assets for wysiwyg iframe content for popup">
     <t t-call-assets="web.assets_common" t-js="false"/>
     <t t-call-assets="web.assets_frontend" t-js="false"/>
+    <t t-call-assets="web_editor.assets_wysiwyg" t-js="false"/>
+    <t t-call-assets="website.assets_editor" t-js="false"/>
     <link rel="stylesheet" href="/website_mass_mailing/static/src/css/website_mass_mailing_popup.css" type="text/css"/>
 </template>