Allow creation of empty MediaWiki: pages

This is useful now since it's possible to disable messages by blanking
them (see bug 14176).

Instead disallow creation of page contents equivalent to the default
message to serve the same purpose as disallowing blanking did (prevent
edits which wouldn't change anything anyway; the edit form is even
prefilled with the default contents).

Bug: 50124
Change-Id: I070036f341b866cda67eb928c5b74ad2ce6c527c
Merge "Allow creation of empty MediaWiki: pages"