[Cashtab] Upgrade cashtab msg bytecount function

Summary:
Legacy function getMessageByteSize is overloaded and designed to handle now-deprecated encrypted msgs. It is also the last holdout still using the legacy generateOpReturnScript, which has been replaced by better functions in the src/opreturn directory.

Write a simpler function to get bytecount for Cashtab msgs. Add same unit tests as old function and some others.

Delete now unused functions getMessageByteSize and generateOpReturnScript.

Implement in Send.js

Test Plan:
npm test
grep -r generateOpReturnScript src/ and no output

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15073