[web] Create a dummy topbar component

Summary:
issue: https://linear.app/comm/issue/ENG-2739/move-navigationpanel-from-the-sidebar-to-a-top-bar
Adding a Topbar component that will contain the AppSwitcher and the navigation info bar. It will be displayed in the Inbox tab.
For now it displays "dummy", because adding it's contents requires a lot of changes to the code. So in the interest of keeping the diff small it has a dummy content for now.

Test Plan: Displayed it in the App component.

Reviewers: przemek, michal, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6341