[ticket/12811] Fix padding/margin bug affecting bottom layout

This reverts the top/bottom spacing of 12px on the body tag back to padding, which how it was always done prior to the [recent change](https://github.com/phpbb/phpbb/pull/2682/files) that moved it to the margins of the wrap tag, which do not work for all browsers.

https://tracker.phpbb.com/browse/PHPBB3-12811

PHPBB3-12811