Add WebMention support

[fluffy-critter]

Expected Behavior

Per the checklist on https://indiewebify.me it would be good to support the WebMention protocol in Publ, both for sending and receiving WebMentions.

Possible Solution

Sending WebMention

There are a few WebMention implementations for Python, of which webmention-tools looks like the easiest to integrate.

Probably the easiest way to do this is whenever an entry gets reindexed, another task should be added to the indexer's work queue that checks for any new URLs and sends the appropriate notification along. We will also want a new database table that tracks sent mentions, just to avoid sending spam (unfortunately this won't help with full-site reindexes due to schema changes or Heroku redeployments or whatever, so maybe this should be configured to also also allow a permanent external database to be a good citizen).

Receiving WebMention

Implement a WebMention endpoint which can be configured to store mentions in various ways. Perhaps this can log to a file and save to a database and provide a simple dashboard or something.

Also per the Heroku thing for sending, this shows a need for an external, persistent database.

There can be a (configurable) hourly maintenance task for sending email notifications of new WebMentions, as well.

Open questions

For receiving WebMention, do we need to provide some sort of moderation interface (ugh) and add the ability to automatically list mentions on the template system? Personally I'd prefer to have it only be a notification system for the author who can then decide whether to float acknowledgments of non-spammy ones to a content update or whatever.

How much of a concern is spam, anyway? Trackbacks were a garbage fire...

[fluffy-critter]

http://webmention.io/ is an existing webmention endpoint that is very easy to integrate. I've already added it to both beesbuzz.biz and publ.beesbuzz.biz, although that only provides part of the equation (namely tracking webmentions). This satisfies the basic criteria of "see where people are talking about my stuff" but still requires logging into a separate dashboard for that and manually promoting mentions to content on my site.

This also doesn't really do anything to make it a turn-key thing for much of the intended audience for Publ (people who just want to get their content online without necessarily having to buy in to a lot of protocol and learning how to program and whatever), and it's also such a rapidly-changing area that I'm not sure how much I want to focus on this just yet.

[fluffy-critter]

One other wrinkle: we can't actually send the pings immediately, since a post might be hidden or scheduled or whatever. This is getting complicated.

[fluffy-critter]

Changing to WONTFIX and closing, per http://publ.beesbuzz.biz/blog/113-Some-thoughts-on-WebMention