Prevent CashTab from creating an SLPA send transaction that sends more than the user's
balance. The balance error will be corrected in bch-api.

Confirm tests still pass:

npm test

Confirm app still runs locally:

npm start

This patch handles a specific edge case where the user has a token with 9 decimal places of precision
and bch-api is returning an off-by-one rounded balance. To test this specific case, create an SLPA
token with 9 decimal places. Send 0.000000001 of it to another SLPA address. Your CashTab balance will not change.
Then attempt to send all of that SLPA token. CashTab will throw an error.