[Cashtab] Match number step input to token decimals

Summary: Form validation already matches token decimals. However, default step for number inputs if not specified is 1. This means a user will see a form error if they try to send a decimal fraction of a token, even though the send will be successful. This diff patches the bug by adjusting step to match token decimal places.

Test Plan: npm start, go to send a token, observe that scrolling over the 'Amount' field increments the value by decimal places of SLPA token to be sent

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8982