[Cashtab] Improve UI for SendToken screen

Summary:
T3491

UI improvements for SendToken screen.

More improvements are still available --- for example, plan to show the whole airdrop form on that switch --- but the changes here are enough for one diff. Still, please comment any further improvements or better ways to implement what is in this diff.

The rest of the Airdrop improvements are pending T3494, then will be in its own diff

Test Plan:
npm test, comment on potential further improvements

flexbox stacks at smaller widths. here's 320px:

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15819