[Cashtab] Simplify and standardize buttons

Summary: Cashtab has gone through many iterations of button design without cleaning up past iterations. Standardize primary and secondary buttons.

Test Plan: npm test, any aesthetic improvements / suggestions

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15681