Implement q=categories for Micropub

To enable clients to i.e. autocomplete categories for tags, we can implement the q=categories endpoint.

This requires a new client, to reach out to the taxonomies.json file that has been newly exposed, taking care to ignore any empty strings, which appear due to my inability to create the JSON array without a trailing comma.

Note that the field used by my site is tags, but Micropub expects it to be called categories, so we need to remap it internally.

Closes #189 (closed).