DefaultSkin form should be accessible

The WCAG automated test highlighted some issues with the page for admins at

`http://localhost:8080/xwiki/bin/edit/XWiki/DefaultSkin`:

• Ensure every form element has a label:
  • #XWiki\.XWikiSkins_0_name
  • #XWiki\.XWikiSkins_0_baseskin
• Ensure select element has an accessible name:
  • #XWiki\.XWikiSkins_0_outputSyntax

This was reported in xwiki.skin.test.ui.AllIT for the method modifySkinObjectTemplate.