Preload abusefilter-warning if created page is blank

This patch makes a new warning message preload the text 
of MediaWiki:abusefilter-warning when the message is created 
through the filter editing view. Generally, it will make 
users more convenient because filter warnings
usually have a pattern, and preloading a pattern from the page
will save time. This patch doesn't work when
MediaWiki:abusefilter-warning hasn't created yet.

Change-Id: Ia327463c52060bfd55c0a1fa6fcec592cc3c3999
Merge "Preload abusefilter-warning if created page is blank"