WebSession: fix mail cookie expiration

* Fixes an issue with mail cookie activation link expiring in
  1 day instead of using the value defined by
  CFG_WEBSESSION_ADDRESS_ACTIVATION_EXPIRE_IN_DAYS.
  (closes #1596)

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>