[FIX] mass_mailing: background snippet option outlook compatible (#20137)

Origin of the fix: the snippet background image did not change the background image.
This was due to commit a14f89c which wraps the snippet's content into a table for outlook display compliance

This issue is fixed here, with the modificaion of the background snippet option.

OPW 772442

addons/mass_mailing/static/src/js/mass_mailing_editor.js

@@ -281,4 +281,15 @@ if ($("#editable_area").html().indexOf('on_change_model_and_list') !== -1) {
     $("#editable_area").empty();
 }
 
+// Adding compatibility for the outlook compliance of mailings.
+// Commit of such compatibility : a14f89c8663c9cafecb1cc26918055e023ecbe42
+options.registry.background.include({
+    start: function() {
+        this._super();
+        var $table_target = this.$target.find('table:first');
+        if ($table_target) {
+            this.$target = $table_target;
+        }
+    }
+});
 });