[web] Create a dummy topbar component
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