Bug 26597: (follow-up) Add disabling / enabled the save button while waiting from...

Bug 26597: (follow-up) Add disabling / enabled the save button while waiting from api to respond

I've updated the test plan since the REST API route takes a little time to respond

To test:
1) Apply patch
2) Search PrefillGuaranteeField preference and make sure some fields are selected
3) Select a user that can have a guarantor
4) In the edit form, click on "Search to add" in "Patron guarantor" fieldset
5) Choose a patron who has at least one of the fields in 1) set
6) Click Select
7) Wait until save button is enabled
8) Confirm guarantee's information is filled from the guarantor's record
9) Check that any preexisting information is not overwritten

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Emmi Takkinen <emmi.takkinen@koha-suomi.fi>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>