[ticket/11984] Prevent AJAX background animation flicker in ACP

PHPBB3-11984
[ticket/11984] Prevent AJAX background animation flicker in prosilver

PHPBB3-11984
[ticket/11984] Fix stray </ul> and active tab hover in ACP

PHPBB3-11984
[ticket/11984] Hide <dfn> for invalid extensions

PHPBB3-11984
[ticket/11984] Do not resize textarea on touch devices

PHPBB3-11984
Merge pull request #1834 from cyberalien/ticket/11984

Move responsive design adjustments