[Cashtab] UI Updates

Summary:
Update to the overall design of Cashtab. Unfortunately it is a rather large diff, but necessary to have a cohesive change.
Majority of edits are just CSS rules with the exception of the added receive screen and layout edits.

Test Plan:
npm start

• make a new wallet and test sending and receiving
• create a token and send it to another address
• test all configuration settings

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, bytesofman

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D10970