This simplifies things overall, and avoids resolve-urls replacing mentions like
'@foo@bar.org' with links to the 'about' page of the instance.
Unfortunately this wasn't easy to add a test for.
When tweeting with an image, twitter will append a link to the image to the end
for the tweet text. That's not useful since we re-upload it to mastodon anyway,
so chop off the link to twitter in that case.
