[web] show total unreads in the navigation sidebar home button

Summary:
This diff introduces the unread badge in the navigation sidebar home button component. Here we want to show the user the total number of unread messages they have on Comm across all of their communities

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

Depends on D10390

Test Plan:
Please see the screenshots below

Reviewers: atul, rohan, kamil

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10391