[web] fixup pinned message modal

Summary:
As I was cleaning up the pinned message modal, I noticed several things not working properly with this modal.

• If you unpinned a message from within the modal, the title of the modal would not update to reflect the correct number of pinned messages in the chat
• If there were no pinned messages in the modal, there was no text/output to indicate to the user that the list was empty

Linear task: https://linear.app/comm/issue/ENG-5945/fix-up-pinned-messages-modal

Depends on D10347

Test Plan:
Please see the demo video below

Reviewers: atul, rohan, kamil

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10348