Allow sys admin to call /c/UserExists
Allow sys admin to call /c/UserExists

The commit fix a regression introduced by #11672 which impacts admin
adding new users.
When admin is creating new users, /c/UserExists is called by UI. We must
allow it called by admin when self-registration is turned off.

Signed-off-by: Daniel Jiang <b8e1ca884a84f218767196db52915b5edb9f310e@vmware.com>

wy65701436/harbor,reasonerjt/harbor,wy65701436/harbor,wy65701436/harbor,wy65701436/harbor,wy65701436/harbor,reasonerjt/harbor,reasonerjt/harbor,reasonerjt/harbor,reasonerjt/harbor,wy65701436/harbor,reasonerjt/harbor