Add support for `client_id` into /mf2/ requests

As part of #25 (closed), it'd be quite useful to start to put data into the /mf2/ data to start understanding more about which client has been used to create the post, so then it can be displayed for users of the site.

To do this, we can add some extra information into our Authentication object, which then stores extra information about the authentication interaction. For now, we'll pull the client_id from the token endpoint, as that's the source of truth for whose token it is.

We also need to make sure that the client is populated in the HugoMicroformats2 object .

Closes #25 (closed).