[Cashtab] Add note to alert users to OP_RETURN msg visibility.

Summary: Added alert to inform users that Cashtab messages are visible on the blockchain. Related to task T2103

Test Plan:
cd web/cashtab && npm start
navigate to send tab and click advanced dropdown
ensure that the alert appears and that messaging is correct.

Reviewers: emack, bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10705