Add support for Micropub

As part of creating more types of content, I want to be able to easily use a Micropub server to push new content to this website.

However, before I can start doing that, I need to publicise my Micropub endpoint. This will be done, as per the Microsub support, using an HTML link element.

As part of this, also mark my www-api as a domain not to check within HTMLProofer, as this will almost certainly error, as these URLs shouldn't be hit!