[FIX] website: prevent default image drag and drop

By default, images on websites can be dragged and dropped in browsers.
As we have a custom drag & drop system, this commit removes the default
drag & drop behavior during the edition of a website page. In addition,
this commit prevents `OdooEditor` from managing drag and drop on images
that should not be dragged and dropped by the user.

task-3369600

closes odoo/odoo#125151

Signed-off-by: Benjamin Vray (bvr) <bvr@odoo.com>

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

@@ -54,6 +54,19 @@ const wSnippetMenu = weSnippetEditor.SnippetsMenu.extend({
         };
         this.$body[0].ownerDocument.addEventListener('selectionchange', this.__onSelectionChange);
 
+        // Even if we prevented the drag via the css, we have to override the
+        // dragstart event because if one of the image ancestor has a dragstart
+        // listener, the dragstart handler can be called with the image as
+        // target. So we didn't prevent the drag with the css but with the
+        // following handler.
+        this.__onDragStart = ev => {
+            if (ev.target.nodeName === "IMG") {
+                ev.preventDefault();
+                ev.stopPropagation();
+            }
+        };
+        this.$body[0].addEventListener("dragstart", this.__onDragStart);
+
         // editor_has_snippets is, amongst other things, in charge of hiding the
         // backend navbar with a CSS animation. But we also need to make it
         // display: none when the animation finishes for efficiency but also so
@@ -70,6 +83,7 @@ const wSnippetMenu = weSnippetEditor.SnippetsMenu.extend({
     destroy() {
         this._super(...arguments);
         this.$body[0].ownerDocument.removeEventListener('selectionchange', this.__onSelectionChange);
+        this.$body[0].removeEventListener("dragstart", this.__onDragStart);
         this.$body[0].classList.remove('o_animated_text_highlighted');
         clearTimeout(this._hideBackendNavbarTimeout);
         this.el.ownerDocument.body.classList.remove('editor_has_snippets_hide_backend_navbar');