[web] Reintroduce AppSwitcher

Summary:
This component renders a reusable component. The reusability will be achieved in next diffs by modifying NavigationPanel so that it receives navigation elements.
Depends on D3338

Test Plan: Check if AppSwitcher continues to work correctly

Reviewers: benschac, atul, def-au1t

Reviewed By: benschac, atul

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3339