[Cashtab] Add privacy policy

Summary:
T2270

Google chrome store requires extensions to have a privacy policy for approval.

This diff creates a privacy policy accessible by a URL in Cashtab. It also adds a link to this policy from the Settings page (Configure.js)

Test Plan:
npm start
Navigate to settings, click privacy policy link at bottom, confirm privacy policy is displayed
Navigate to localhost:3000/#/privacy, confirm privacy policy is displayed
Open an incognito window and navigate to localhost:3000/#/privacy and ensure privacy policy is displayed (and menu is not, as no wallet has been created)
Test in extension (just as a link from the settings page)

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D11110