Add plugin to send webmentions after deployment

[bentasker]

This adds a plugin to detect external links in posts/pages and perform WebMention (the modern form of pingback) endpoint discovery on them in order to send a WebMention notification.

[Kwpolska]

@bentasker Can you fix the code style violations? (Running black on your code should be enough to resolve those complaints.)

[bentasker]

Sure, done

[Kwpolska]

Thanks for your contribution!

[bentasker]

Thanks!