CIWEMB-476: Usability improvements to the "manage instalments" forms

[omarabuhussein]

Before

Issue 1

When trying to add a line item through the manage instalments screen, and after clicking "Apply", the "Apply" button will remain enabled until the form is fully submitted, which might lead to users clicking on it twice or more by mistake, which may result in multiple submissions.

Issue 2

When trying to add a line item through the manage instalments screen or if you are using the "Switch membership type" option, and if you select a "one-off payment" option and entered a non numeric value, then the payment form will disappear and nothing will happen:

After

Issue 1

The "Apply" button will be disabled after the user clicks on, and so preventing multi submissions:

Issue 2

Entering non numeric value in the amount field will show the validation error properly.