BUG LeftAndMainSubsites::Subsites() fails with limited CMS access

Users with non-ADMIN permissions won't see the dropdown of available
subsites, because LeftAndMainSubsites::Subsites() will check if
the user has a non-existent code CMS_ACCESS_CMSPagesController.

Rather than figuring out the code using the extension owner class,
just use the required_permission_codes static available on CMSMain
and subclasses.

If no permission is found, it defaults to CMS_ACCESS_CMSMain