[ticket/11935] Fix invalid HTML and Errors

```<label>``` tag can only contain one form element. Most files handle this correctly, but a few I found still had the ```<input>``` tag included with the ```<select>``` tag inside a ```<label>``` tag.
Removed obsolete table attributes (cellpadding, cellspacing), they are handled by CSS
Various fixes to the Extension List in the ACP

http://tracker.phpbb.com/browse/PHPBB3-11935

PHPBB3-11935