Proposal: a citation dashboard, if one is ever wanted

[dprada]

serve_ui was removed in uibcdf/ackredit#57: an HTTP server inside a citation library is
surface with an indefinite maintenance cost, and it was never finished. This records what a
dashboard would need if it is ever wanted, so the decision is not re-made from scratch.

It does not belong in this package. uibcdf/ackredit#36 built the ackredit.formats
entry-point group, so a separate distribution can register a format and serve it without
Ackredit depending on a web framework. That is the shape to reach for.

What the removed one lacked, and what a real one needs:

• a way to stop it. It started a daemon thread and nothing took it down. The same gap
  was closed for the import hooks and the exit reminder in uibcdf/ackredit#34, and a
  server is the case where it matters most.
• a way to know it came up. It printed a URL and returned, so a caller could not tell a
  bound port from a failed one.
• tests that serve. The only test it had was that it names Flask when Flask is absent;
  nothing ever asked it for a page.
• a decision about what it shows. A live view of a running session is a different
  question from a report of a finished one, and only the second is what Ackredit produces
  today.

The escaping it needed already exists and stays: ackredit/formats/_html.py and
_links.py, from uibcdf/ackredit#25.