[IMP] mass_mailing: do not load images on add sidebar and commitChanges

Images in mass_mailing are not lazy loaded, which is deliberate for the
mailing itself and its conversion, but when we load the sidebar we add
the HTML to all the snippets and hide it so if these images are not lazy
loaded, they all get loaded along with the sidebar, which is slow and
unnecessary.

This loads the images only once we add them to the mailing, which is
when we remove the lazy loading attribute.

task-3323894

Part-of: odoo/odoo#126845

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

@@ -535,6 +535,7 @@ export class MassMailingHtmlField extends HtmlField {
         for (const img of $container.find("img")) {
             const $img = $(img);
             const src = $img.attr("src");
+            $img.removeAttr('loading');
 
             let m = src.match(/^\/web\/image\/\w+\.s_default_image_(?:theme_[a-z]+_)?(.+)$/);
             if (!m) {

addons/mass_mailing/static/src/js/snippets.editor.js

@@ -32,11 +32,26 @@ export const MassMailingSnippetsMenu = snippetsEditor.SnippetsMenu.extend({
             this.$editable = this.options.wysiwyg.getEditable();
         });
     },
+    /**
+     * @override
+     */
+    callPostSnippetDrop: async function ($target) {
+        $target.find('img[loading=lazy]').removeAttr('loading');
+        return this._super(...arguments);
+    },
 
     //--------------------------------------------------------------------------
     // Private
     //--------------------------------------------------------------------------
 
+    /**
+     * @override
+     */
+    _computeSnippetTemplates: function (html) {
+        const $html = $(`<div>${html}</div>`);
+        $html.find('img').attr('loading', 'lazy');
+        return this._super($html.html().trim());
+    },
     /**
      * @override
      */