Bug 32925: Display loading info when a form is submitted

Bug 32925: Display loading info when a form is submitted

With this new ButtonSubmit component all our submit buttons will look
and behave identically.
Here we need a "submitting" information to be displayed, and the form to
be "deactivated".

This patch is a POC and use the new component on the add agreement and
delete agreement forms

Test plan:
It's easier to test if you add a sleep in the relevant place
(Koha/REST/V1/ERM/Agreements.pm for list and add/update)
Confirm that you see a "Submitting..." in the middle of the screen

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>