[Cashtab] Bring extension in line with latest prod version

Summary:
T1727

Cashtab has gone through a number of significant refactors to better standardize how the app handles its loading state.

This diff brings the chrome extension up to date with these updates.

Code comments and organization are added to src/components/App.js to improve readability when comparing to extension/src/components/App.js

Test Plan:

1. Compare the differences between src/components/App.js and extension/src/components/App.js in a diff checker (diffchecker.com works well enough) to establish how the prod version of the app differs from the extension. There are only two changes: (1) Prod version includes a test feature enabled on wallets with a balance in a certain eToken, and (2) the extension replaces the top right logo with a link to open the extension in its own tab.
2. npm run extension
3. Navigate to chrome://extensions in Chrome or brave://extensions in brave
4. Enable developer mode and select Load unpacked, load the folder web/cashtab/extension/dist
5. Verify that the extension loads and performs like the prod app. Try sending and receiving some txs. The code is almost entirely identical to the prod app so step (1) above is probably more informative

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9927