Add support for FB messenger 'pinned' messages

[0xdevalias]

You can pin individual messages, including links and photos, in a chat to reference them later. Pinned messages are pinned for everyone in the chat.

To see all pinned messages in a chat, tap on the chat name at the top of a chat, then tap Pinned Messages. Messages are listed from most to least recently pinned. You can tap on individual messages here to jump to the message location in the chat, or tap and hold on individual messages to unpin them.

• https://www.facebook.com/help/messenger-app/1335189030292814
  • https://www.facebook.com/help/messenger-app/1335189030292814?cms_platform=iphone-app&helpref=platform_switcher
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