New setting: wgCascadingRestrictionLevels

Change I5f8bcc89 introduced a new setting, $wgCascadingRestrictionLevels.
This change implements it in the Wikimedia wikis' configuration.

Goal .............. Define the levels of a cascading protection
Default value ..... sysop (current behavior)
Custom values ..... pl: editor, sysop (per bug 48374 request)

Change-Id: I91826148149b86413a7a6a2e20d21289d79b52bc