[Cashtab] Add social links to Settings page

Summary:
T3404

Add Twitter and Facebook links to the bottom of the Settings Page

Test Plan: npm start, click hamburger menu, confirm icon looks ok, confirm link opens in a new tab

Reviewers: #bitcoin_abc, emack, Fabien

Reviewed By: #bitcoin_abc, emack, Fabien

Subscribers: emack, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15174