Set the default database schema to "mediawiki" so as not to break the CLI installer.

Due to changes made to support Microsoft SQL Server, $wgDBmwschema changed its default from
"mediawiki" to null in DefaultSettings.php, as anything else horribly broke every DBMS that did
not use schemas (such as MySQL and SQLite). This change makes it so that the default value can
be properly overridden again by PostgreSQL and Microsoft SQL Server, and also enables the
--dbschema flag to the CLI installer.

Bug: 64043
Change-Id: Id364306d883e0d494b948854e05f3f79ba7dd6d2