[web] Display the number of pinned messages in a banner across chat

Summary:
Each thread that has pinned messages should display a banner across the top indicating the count.

Linear: https://linear.app/comm/issue/ENG-3448/display-the-number-of-pinned-messages-in-a-banner-across-chat

Depends on D7309

Test Plan:
Check threads that have 1) 0 pinned messages, 2) 1 pinned messages, 3) >1 pinned messages

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7310