Allow ‘ADMIN’ and ‘CMS_ACCESS_LeftAndMain’ access to CMS. Fixes CWPBUG-113.

Previously, only the global ‘ADMIN’ permission was allowing users to bypass the
stricter Permission check. We also need to allow the ‘CMS_ACCESS_LeftAndMain’
permission to bypass this check, as otherwise a user who is in a Group with the
‘Access to all CMS sections’ permission set (which only sets the
CMS_ACCESS_LeftAndMain permission code and no others) would be denied access to
the CMS for that sub site.
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.
Merge pull request #130 from madmatt/pulls/permission-fix

Allow ‘ADMIN’ and ‘CMS_ACCESS_LeftAndMain’ CMS access. Fixes CWPBUG-113