Retrieve RSVPs to my site via `q=source` + Log cite upserts in stubbed repository

Now I post events to my site that are retrievable by my Micropub server, I should speed up my ability to RSVP by using q=source on the URL, instead of waiting for it to be published.

I've decided to leave this as an HTTP request, instead of using the repository as this right now is quite an HTTP-specific class, and until we refactor into the strategy pattern in #281, it makes sense to leave it as this.

As the logic is getting a little complex, we extract out to a new retrieveHevent method, which allows us to handle different cases more easily, including how the Micropub q=source response is the item itself, not the items that is usually returned from parsing an object with a Microformats library.

We need to add the ObjectMapper dependency to make sure that we can create an expected JsonNode.

Closes #212 (closed).