Smilie sticker extension/pack

[nolanw]

This was a thing, then I got rid of it because of some code signing snafu I didn't want to properly fix. That had been a "static" sticker pack (just a bunch of image files dragged in to an asset catalog), but ideally we wouldn't duplicate all those images so I wanted to do a proper sticker extension that could share resources with the app. Then I didn't do that, so we're entirely without a sticker pack right now.

It would also be great if the sticker extension/pack was usable without signing in to the app.