[FIX] mass_mailing: ensure design element changes are applied

When we change something in the DESIGN tab of mass_mailing, we edit a
stylesheet. This means we need to flush the rules cache so
convert_inline applies the new rules in its conversion.

opw-3114599

closes odoo/odoo#111778

X-original-commit: 4ebf106e65ae4901c03c76606a05d8cd2e0790d7
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/static/src/js/mass_mailing_snippets.js

@@ -220,6 +220,9 @@ options.registry.DesignTab = options.Class.extend({
             cssTexts.push(rule.cssText);
         }
         this.styleElement.textContent = cssTexts.join('\n');
+        // Flush the rules cache for convert_inline, to make sure they are
+        // recomputed to account for the change.
+        this.options.wysiwyg._rulesCache = undefined;
     },
     /**
      * @override