Add class name to protected pages, to allow for specific CSS styling of edit area

Also get rid of checking for NS_MEDIAWIKI explictly and use
MWNamespace::getRestrictionLevels instead

Bug: 50783
Change-Id: I5986ddb9b6f17e4a2aca12dbb551cce4a6cfd663
Merge "Add class name to protected pages, to allow for specific CSS styling of edit area"