Bug 16458: Update library when a guarantor is set

Bug 16458: Update library when a guarantor is set

If you edit a patron from the Edit link of the patron information block
(step 1 of the memberentry script), the branchcode is not displayed. If
you set a guarantor to a child from this form, a JavaScript error will
be raised:
  JavaScript error form.branchcode is undefined

It happens because there is the branchcode is not displayed and there is
no element with a branchcode name.

To avoid this issue, a branchcode hidden input is added to the form and
it will be updated when a guarantor is selected (same behavior as when
you edit a patron using the complete form).

Test plan:
Edit a patron using the Edit link of the patron info block (not the Edit
button in the toolbar)
Set a guarantor
=> Notice that the branchcode of the patron has been updated with the
branchcode of the guarantor

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>