Added site options to allow for override of user expiry when
setting site expiry (if the user is not site admin).

- for new users
- for new users and existing users without expiry set
- for all users

Allowing for switching back to 'no end date'.
Need to force refresh the page to display info correctly after save.

Change-Id: I0a772b3db7c2ae5144ed4120cf851d4bba633f66
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>