Combine Indie post types into Microformats2 content type

As part of #572 (closed), I wanted to experiment with having all the Indie content types (i.e. reply, bookmark, like).

This is twofold - firstly it was driven from the work to build my Micropub endpoint, as it won't need some logic in there to determine the correct content type to create within my site. But it was also to remove the weird state where I was partly writing Microformats2-like properties for my content, but also not very, because it was my own setup around it as i.e. I didn't want to have to worry about doing that mapping.

This required quite a few changes, to reformat the existing content into the new Microformats2 format.

We remove schema validation for the new content type as we want to be doing that validation through the Micropub endpoint rather than in this site.

Finally, we move the RSVP calendar to a custom output format, as we can't use it under the RSVPs content type, as that doesn't exist any more.

Additionally, resolve a bug with how datetimes are rendered.

Closes #572 (closed), #500 (closed).