Unbreak cascading protection

Title::getRestrictions can return "sysop", which will not match if
replaced out of $cascadingRestrictionLevels but not $editrestriction,
making it impossible to cascade protect a page with the default value
of $wgCascadingRestrictionLevels.

Follows-up I6bf650a3fbdab8589ae6945c8c916eafd949e41c.

Change-Id: Ie7183ed9509ea9c65a7197fc9cead6c8423b852e
Merge "Unbreak cascading protection"