Bug 12648: Refactoring to prepare user search for reuse

Bug 12648: Refactoring to prepare user search for reuse

A previous enhancement allows to link basket with patrons.
Next patches will use the same way to link order with patrons.

In order to avoir c/p of code, this patch refactores this part of code.

Test plan:
1/ Verify there is no regression on adding/modifying users to a basket.
(acqui/basket.pl?basketno=XXX, "Managed by", "Add user").
2/ Note that you get a friendly message if the user is already present in the
list and when the user has correctly been added to the list.
3/ Note that the list uses the member search service (ie. DataTable +
serverside processing).

Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>