[web] show unreads for each community in the community list

Summary:
This diff introduces the unread badge to the community list in the navigation sidebar. Here we want to show the unreads broken down by each community

Linear task: https://linear.app/comm/issue/ENG-5951/introduce-a-new-unread-badge

Depends on D10435

Test Plan:
Please see the screenshots/demo video

Reviewers: atul, rohan, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10393