Show pinned messages - New UI

[cammellos]

As a user I want to be able to see pinned messages So that I know the important information about a community/channel

This needs to be implemented for 1-to-1, private group chats & communities.

Designs

https://www.figma.com/file/PPWkgOYlZZDxZv5SDGsZVV/Posts-%26-Attachments?node-id=3425%3A413546

This includes showing the banner on top with the latest pinned message, the number of pinned messages, and showing the pinned messages https://www.figma.com/file/PPWkgOYlZZDxZv5SDGsZVV/Posts-%26-Attachments?node-id=3505%3A368386

Depends on: #14022

Component dependency: https://www.figma.com/file/WQZcp6S0EnzxdTL4taoKDv/Mobile-Components?node-id=863%3A26323 Banner component