[Cashtab] Support token sends from send screen

Summary:
Improve the Send screen so a user can select and send a token from that screen.

This is step one toward more flexibility in bip21 token sends, as well as more powerful token sends (including msg, one to many)

Things we can do after this diff

• Modify bip21 so that token sends populate this token form instead of shimming token sends into a special bip21 form
• Cashtab support bip21 token sends without a quantity param
• Add support for bip21 empp_raw to support ALP sends with data pushes

Test Plan:
npm test

this diff is deployed to cashtab.io so you can check it out there

Reviewers: #bitcoin_abc, johnkuney

Reviewed By: #bitcoin_abc, johnkuney

Subscribers: Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D19205