[Cashtab] Fix fee calculation for one to many txs

Summary:
T2268

Fee calculation is based on 2 outputs. However, for multi-send txs, fees need to take into account the total number of outputs.

Test Plan: Generate a multi-send tx with 20+ outputs. Send successfully with no fee error.

Reviewers: emack, #bitcoin_abc

Reviewed By: emack, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11144