Provide more useful information about webmention sending

Given a URL:
- Look for h-entry markup, if none found point to previous stage
- If h-entry markup found, find all links in summary and content as well as in-reply-to links
- Maybe also look for rel-in-reply-to
- Return HTML page listing found links, in JS fire off calls to send webmentions
- Next to links, return whether or not they support webmention/pingback, what the discovered endpoint is, whether or not the webmention succeeded

Some of this is probably unnecessary to begin with, e.g. the async part (which would also require the endpoint to be split up into find-links and send-webmention), so this is not high priority.

Start with the existing URL-processing flow and the webmention endpoint behavior described in the issue. Trace how h-entry links, in-reply-to links, discovered endpoints, and send results can be represented in the returned HTML and JavaScript; done means the page lists the relevant links and reports endpoint discovery and sending outcomes.

Written by the indexing model from the issue text.