[web] factor out pinned messages banner into its own component

Summary:
We will be using the thread top bar component as the panel header for the message list panel item. In order to accomplish this we will need to decouple the pinned messages banner from the top bar. To make the move a bit easier/more organized, I took a second to make the pinned messages banner into its own component.

Additionally, I snuck in some color changes that have the pinned messages banner utilize the new color design system.

Test Plan:
flow and confirmed the pinned messages banner still works as expected. Also please see the screenshots below:

dark:

light:

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10548