Determine links manually

The existing solution using /mentionall on mention.tech unfortunately wasn't quite up to scratch for sending Webmentions, as it required specific markup to pick up all the links, whereas I'd want to have all of the links sending notifications.

To make this work more easily, we can use the Jsoup library to perform the heavy lifting and parsing of HTML.

We use a JsoupConnector as a way to make the unit testing easier, as then we don't have to deal with calls to a static method, but can instead use this wrapper.

Additionally, we don't want to send Webmentions to relative URLs (as they don't seem to work anywhere) as well as reduce the amount of traffic we're sending to https://mention.tech.

Follow-up to #8 (closed).