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.

Fallback to checking required_permission_codes first, and failing
that, check for CMS_ACCESS_LeftAndMain