[Cashtab] Move parseOpReturn and unit tests to cashMethods

Summary:
As per T2101, moved parseOpReturn function and corresponding unit tests to cashMethods.
This is a prerequisite for T2015.

Test Plan:
npm start
test with an outbound normal message with a single recipient tx and message label shows cashtab message
test with an outbound normal message with a multi recipient tx and message label shows cashtab message
test with an outbound encrypted message with a single recipient tx and message label shows cashtab message
test with an inbound normal message from electrum with a single recipient tx and message label shows external message
test an outbound long message
test an outbound message with special symbols
ensure correct styling for encrypted and unencrypted messages

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10829