[web] Add danger zone page

Summary:
Add new page and connect it to the route so that it is displayed when a navigation item is selected.

Depends on D4093

Test Plan: Open danger zone and check if the header is displayed.

Reviewers: def-au1t, atul, benschac

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4094