[Cashtab] [p1] Patch fee falc in sendToken

Summary:
T2644

Part 1 of a stacked diff to clarify procedure and patch fee calculation in sendToken

Rename inputUtxos to xecInputUtxos to reduce ambiguity

Test Plan: npm test, confirm no instances of inputUtxos missed

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11924