[web] Move AppSwitcher to web/topbar

Summary:
issue: https://linear.app/comm/issue/ENG-2739/move-navigationpanel-from-the-sidebar-to-a-top-bar
Moving the AppSwitcher from web/sidebar to web/topbar becaue it will be used only in the top bar in the near future.

Test Plan: Run the web app, check that the AppSwitcher works properly.

Reviewers: przemek, michal, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6342