Yet another issue with Bleach 3.0's linkification: when used via the
filter as part of sanitization (which is necessary right now due to
*another* issue where it escapes valid HTML tags), it doesn't properly
linkify urls that contain an ampersand.

As a (temporary?) workaround, this stops using Bleach's linkification
entirely and switches to cmark-gfm's "autolink" extension. These aren't
perfectly equivalent, and the switch results in two other issues that I
consider more minor than links including ampersands not working:

- autolink will initially create links for ftp:// urls and email
  addresses. The final sanitization will remove these links due to the
  protocol whitelist, but it will leave behind a bare <a> tag. So the
  text will *appear* linked but not actually link to anything. If I
  decide to stick with autolink, it should be pretty straightforward to
  fix this by stripping all bare <a> tags from the final HTML.
- autolink doesn't create links for bare domains. For example, writing
  "example.com" won't result in a link, it's necessary to write
  "www.example.com" or include a protocol like "http://example.com".