[FIX] mass_mailing: switch to theme images at 1st theme selection

Without this patch, default images for themes are not used automatically
in themes' templates when one is chosen for the 1st time. This was not
inducing any bug in standard 10.0 but with custom-made templates, you had
to put the correct themes images in the defined template. This allows to
just `t-call` snippets instead of having to fully hardcode them in theme
templates.

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

@@ -326,7 +326,6 @@ snippets_editor.Class.include({
             switch_theme.last = theme_params;
 
             $body.removeClass(all_classes).addClass(theme_params.className);
-            switch_images(theme_params, $editable_area);
 
             var $old_layout = $editable_area.find(".o_layout");
             // This wrapper structure is the only way to have a responsive and
@@ -349,8 +348,9 @@ snippets_editor.Class.include({
                 $contents = $editable_area.contents();
             }
 
-            $editable_area.empty().append($new_layout);
             $new_wrapper_content.append($contents);
+            switch_images(theme_params, $new_wrapper_content);
+            $editable_area.empty().append($new_layout);
             $old_layout.remove();
 
             if (first_choice) {