[landing] Add "launch app" icon to Header

Summary:
Adding an icon so the web app (https://web.comm.app) can be launched from the landing page.

Note: Won't land this diff until https://web.comm.app is live

Test Plan:
Here's what the header looks like after:

Reviewers: benschac, def-au1t, palys-swm, varun, ashoat

Reviewed By: benschac, palys-swm, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3390