[Cashtab] Improve nav buttons

Summary: Reported issue that the nav buttons in bottom footer are glitchy. The responsive click area is not consistent with the cursor:hover display. This is because they were implemented as TabPanes in antd. This diff removes all Tab and TabPane components + associated css over-rides and replaces with button components, which have a responsive click area that's easier to control.

Test Plan: This is a pure css change. npm start and verify that buttons look okay. Click between them. Confirm functional and no "misclick" issues as with current app (where cursor appears as pointer but does not activate button). npm test to confirm no broken unit tests.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9282