Use HTMLForm to generate the form on Special:ListFiles

- Makes the code easier to read
- This was actually the last usage of Xml::buildForm() in core
- Removed tabindex; not needed since the navigation is already
  in DOM order

Change-Id: Ie0a2819fdf28f734a8c68246be693de7259b8332
Merge "Use HTMLForm to generate the form on Special:ListFiles"