[FIX] web_editor, website: consistently apply undo on CTRL-Z

This fixes a bug in Website, that can be reproduced like so:
- insert a snippet
- insert a char in it
- 2x CTRL-Z
The second CTRL-Z didn't do anything because Summernote moved the range
to the header, which is not editable. This fix moves the range to the
first editable element in the editor in these cases, thereby keeping the
focus in an editable area, and ensuring that hotkeys still function.

The history feature being non-tested as of this version, no test could
be added for this fix.

closes odoo/odoo#34834

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/web_editor/static/src/js/wysiwyg/plugin/history.js

@@ -19,6 +19,12 @@ var HistoryPlugin = Plugins.history.extend({
             console.error(e);
         }
         this.context.invoke('editor.focus');
+        var newRange = this.context.invoke('editor.createRange');
+        if (!this.options.isEditableNode(newRange.sc)) {
+            var firstEditableSel = '[contenteditable="true"]:not(":hidden")';
+            var sc = this.$editable.find(firstEditableSel)[0];
+            this.context.invoke('editor.setRange', sc, 0, sc, 0).select();
+        }
     },
     /**
      * Clear the history.