Bug 27967: Add min attribute to borraccount modals

Bug 27967: Add min attribute to borraccount modals

This patch adds a min attribute to the form fields of the borrower
account page modals to prevent negative values being entered
inappropriately.

Test plan
1/ Trigger the refund modal from the borrower account page.
2/ Attempt to enter a negative value for the amount to refund and note
with the patch applied you face a validation error (without the patch,
upon submission of a negative value you will be faced with a server side
error)
3/ Trigger the discount modal from the borrower account page.
4/ Attempt to enter a negative value for the amount to discount and note
with the patch applied you face a validation error (without the patch,
upon submission of a negative value you will be faced with a server side
error)
5/ Signoff

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>