[web] Implement danger zone UI

Summary:
Add all the components to the page. The design https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1272%3A87631 doesn't specify the copy below the button, but I think it might be a good idea to have a warning there - the same we currently have in delete tab of account settings.

Depends on D4094

Test Plan: Open http://localhost/comm/settings/danger-zone/ and check if it is displayed correctly.

Reviewers: def-au1t, atul, benschac, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4096