[web] Use app switcher in the topbar
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