The discord backend introduced in #279 fails with the rather mysterious {"embeds": ["0"]} when the url starts with the perfectly valid http://bugsink:8000.

We should try to avoid sending the url field for URLs that Discord believes to be invalid. Unfortunately, I can't find the documentation Discord-side that would state what (presumably incorrect) validation they apply to urls.

We could try to reverse-engineer the correct answer by trying a number of URLs though.