Enable RSVPs from Eventbrite

Now I have created eventbrite-mf2 (https://www.jvt.me/posts/2019/10/19/microformats-eventbrite/) I should integrate with it to allow me to RSVP to events on Eventbrite from my Micropub endpoint.

This also required a little test cleanup to rename incorrect references to eventbrite, so they wouldn't fail tests for the wrong reasons.

Closes #58 (closed).