[FIX] website: edit menu not showing url & mailto prefix with '/'

Before this commit:
1. you could not edit a menu URL because the input was hidden.
   This was due to radio options being switched (new page is now selected choice)
   when you create a menu.
   But when you edit, there is no choice and the input should be shown.
2. if you set a menu url to an email address, the display link would be prefixed
   by a / and so it would open mysite.com/mailto:XXX instead of just the mailto

Now:
1. Input is now hidden only when you add a new menu, not when editing a menu.
2. mailto link now works as expected: it will open your desktop mail software

addons/website/models/website.py

@@ -714,8 +714,9 @@ class Menu(models.Model):
         else:
             url = self.url
             if not self.url.startswith('/'):
-                if '@' in self.url and not self.url.startswith('mailto'):
-                    url = 'mailto:%s' % self.url
+                if '@' in self.url:
+                    if not self.url.startswith('mailto'):
+                        url = 'mailto:%s' % self.url
                 elif not self.url.startswith('http'):
                     url = '/%s' % self.url
         return url

addons/website/static/src/js/menu/content.js

@@ -261,11 +261,11 @@ var MenuEntryDialog = widget.LinkDialog.extend({
      */
     start: function () {
         var self = this;
-        self.$('#o_link_dialog_url_input').closest('.form-group').hide();
         this.$('.o_link_dialog_preview').remove();
         this.$('.window-new, .link-style').closest('.form-group').remove();
         this.$('label[for="o_link_dialog_label_input"]').text(_t("Menu Label"));
         if (this.menu_link_options) { // add menu link option only when adding new menu
+            self.$('#o_link_dialog_url_input').closest('.form-group').hide();
             this.$('#o_link_dialog_label_input').closest('.form-group').after(qweb.render('website.contentMenu.dialog.edit.link_menu_options'));
             // remove the label that is automatically added before
             this.$('#o_link_dialog_url_input').parent().siblings().html('');