[Cashtab] Standardize padding and wrapping of app components

Summary:
T3494

Stop using the weirdly named and ubiquitous SidePaddingCtn

Instead, wrap all components in the same container in App.js

We also stop using <br/> as a hacky way to add standard top-spacing in components that lack a customized wrapper.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15821