Check for $wgAllowUserCss and $wgAllowUserJs in EditPage.php

This change checks for the truth values of $wgAllowUserCss and
$wgAllowUserJs before displaying the system/preview messages
userjsyoucanpreview, usercssyoucanpreview, userjspreview and
usercsspreview; if false, it will simply display the standard
messages used by other wiki pages.

Change-Id: I6a5fa8b29010f26435125479e72c423b5e4290f8
Merge "Check for $wgAllowUserCss and $wgAllowUserJs in EditPage.php"