[web] Add logout button

Summary:
Add logout button and make sure that layout remains correct even if username is long
https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1272%3A88750

Depends on D3321

Test Plan:
Check if the page renders correctly.
Modify username using dev tools and make it longer - check if it is rendered correctly.
Click logout button - user should be logged out.

Reviewers: benschac, atul, def-au1t

Reviewed By: benschac, atul

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3322