[web] Use app switcher in the topbar

Summary:
issue: https://linear.app/comm/issue/ENG-2739/move-navigationpanel-from-the-sidebar-to-a-top-bar
Use AppSwitcher in Topbar. Now, that the AppSwitcher is inside of the Topbar, it can be made horizontal, and it can use the styles for a horizontal switcher. The styles for icons in the left-layout-aside.css are no longer needed,
because the other switcher doesn't display any icons.

Test Plan: use Topbar in App component, check that it displayes correctly.

Reviewers: przemek, michal, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6345