Bug: [Simple Taxes] It is possible to set negative value on tax rate.

[eCzerniak]

What are you trying to achieve?

Create new tax rate

Steps to reproduce the problem

1. go to taxes in configuration
2. set in countries - in e.g US -default rate eq to "-20"
3. create checkout

What did you expect to happen?

It should be not possible to set tax rate as a negative value.

Logs

Environment

Saleor version: … OS and version: …