Add support for pinning chat messages
Description of the new feature
Pin a specific message/image to the top of the chat window to make it available for quick access. Use cases include:
- pinning an address
- pinning a schedule
- pinning a link to a pad / a share / a project
For group chats, we could re-use the subject for this, which would be displayed to all participants (synchronized). For 1:1 chats, this could be any message (not synchronized).