[web] Add a badge on the Inbox button

Summary:
issue: https://linear.app/comm/issue/ENG-3178/add-a-badge-on-the-inbox-all-communities-button
This code is effectively copied from app-switcher.react.js, where the badge exacly like what we want here already exists (that badge will be changed)

Test Plan: rub web app, checked that the badge is visible in all tabs but the Calendar tab, and displays the correct number of unreads

Reviewers: michal, kuba

Reviewed By: michal

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6978