[Cashtab] Add deprecation announcement for encrypted msgs

Summary: Show an alert msg on the Send screen for planned deprecation of encrypted msg sending

Test Plan: npm test, review screenshot for date and appearance of notice

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14589