[Cashtab] Improve UX for sending cashtab msg

Summary:
T2207

Stop using the confusing below-send collapse for a cashtab msg. Instead, use a simple switch to show/hide the form.

Put cashtab msg into formdata state object instead of maintaining its own confusingly named state var (confusing bc now cashtabmsg is not the only type of opreturn msg we could expect to send)

Test Plan:
npm test

This diff is live at https://cashtab-local-dev.netlify.app/

note: there is a pre-existing bug where the send button will not be disabled on multisend if there is valid input present in send to one

error and count if error msg is more than 1 line

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15844