[FIX] web_editor: support of SVG elements in snippets

Before this commit, it was not possible to save a page
containing a snippet with a SVG tag.

To support SVG elements, we need to use 'Element.getAttribute'
and 'Element.setAttribute' as mentionned in the documentation:
https://developer.mozilla.org/en-US/docs/Web/API/Element/className

For more information, see GH-14485

addons/web_editor/static/src/js/rte.js

@@ -364,8 +364,8 @@ var RTE = Widget.extend({
                 var $el = $(this);
 
                 $el.find('[class]').filter(function () {
-                    if (!this.className.match(/\S/)) {
-                        this.removeAttribute("class");
+                    if (!this.getAttribute('class').match(/\S/)) {
+                        this.removeAttribute('class');
                     }
                 });