Added support for 'borderless' images. Basically images that we'd like to render
as if they were stickers, even though they're not stickers. On iOS, this will be
stuff like memoji and bitmoji. On Android, in my initial pass, I've just added
support for Giphy stickers. However, we can also detect bitmoji and keyboard
stickers in the future. This is kind of a 'best effort' thing, so as long as we
support receiving, we can just add sending support for more things as we go.
