Add support for cursor-based pagination

[paulrobertlloyd]

The Microsub specification uses cursor-based pagination. This has inspired a similar pagination extension for Micropub.

This PR replaces the previous page-based pagination with this cursor-based method. This solves a few things:

• no longer need to provide a private _count value in Micropub responses
• no longer need to address issue of long list of pages in pagination component
• adheres to existing community standards and conventions

And of course this means one small part of the Microsub spec has already been built.