Add endpoint configuration

[fluffy-critter]

There are alternate endpoints that support the webmention.io API, such as mention.tech. Make it easier to support them!

[Ryuno-Ki]

Perhaps @kevinmarks could help by documenting the API?

So far it looks like https://mention.tech/listmentions?target=<url_encoded_domain>&json=on
(Now redirect to HTTPS version, too).

Its repo declares it as „webmention experiment”: https://github.com/kevinmarks/mentiontech

[Ryuno-Ki]

If we refactor

webmention.js/static/webmention.js

Lines 415 to 419 in 1aff7d3

	let apiURL = `https://webmention.io/api/mentions.jf2?per-page=${maxWebmentions}&sort-by=${sortBy}&sort-dir=${sortDir}`;
	pages.forEach(function (path) {
	apiURL += `&target[]=${encodeURIComponent('http:' + path)}&target[]=${encodeURIComponent('https:' + path)}`;
	});

into its own function, it could be made parametrised.
That would like work via additional data-attribute on the script tag.