Add $wgCascadingRestrictionLevels

A page can only be protected with cascading protection if the
requested restriction level is included in this array.

This replaces previously hard-coded values of 'sysop' and 'protect'.

This is necessary, because if any protection could be
cascading, users could who cannot normally protect pages could
"protect" them by transcluding them on protected pages they are
allowed to edit.

Bug: 47617
Change-Id: I5f8bcc899b46d466161894606cd27bf3b8624bd0