BUGFIX: permissions to check the ‘CMS_ACCESS_LeftAndMain’ global permission.

‘CMS_ACCESS_LeftAndMain’ is used by the PermissionCheckboxSetField to allow
applicable Members to access all CMS sections. There are then further
permissions to restrict the Members (e.g. ‘CMS_ACCESS_LeftAndMain’ will give you
access to the ‘Pages’ section, but you still need the ‘Edit any page’ permission
to actually edit anything).

This patch ensures that the subsites module follows those permissions, and
doesn’t unnecessarily deny permission to legitimate users.