Also added some hardening to make sure that it's impossible for any link
previews to be fetched if the setting is disabled (this was already the
case in practice, we just have some assertions in there now).
Fixes#9956
Also updated the date format -- funnily enough Android will work with
either Z or X in the format, but the test JVM will fail if it doesn't
use X. X is definitely the correct thing to use based on the Javadoc, I
think Android's implementation is just a little more lenient.
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.
