[Cashtab] [deprecate etoken addresses p1] Support token sends to ecash: addresses

Summary:
T2802

Allow users to send etokens to ecash: addresses

Test Plan:
npm start
Send an etoken tx to a Cashtab wallet's ecash: address
Send an etoken tx to a Cashtab wallet's etoken: address
Confirm that validation works for invalid etoken: and invalid ecash: addresses

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12677