[web] introduce unread badge component

Summary:
This diff introduces a new unread badge component. Our previous unread badge component was built inline; however, since the unread badge will be used in several places in the new web app redesign I wanted to create a simple component that we can use anywhere.

Here's a screenshot showing the unread badge component in the figma:

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

Depends on D10364

Test Plan:
Please see the screenshot below (the 99+ is just dummy data to test if I have more than 99 unreads then the unread badge would say 99+)

Reviewers: atul, rohan, kamil

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10390