A better permalinks / rewrite rules UI

We should evaluate and consider adding the WP Permastructure plugin I wrote years ago - it might be a bit in need of updating but the rewrites in WP have barely changed.

https://github.com/interconnectit/wp-permastructure

The core idea of what it adds is the key thing, it extends the permalinks UI to allow users to edit the permalink structure for all custom post types and taxonomies, adding taxonomy rewrite tags, as well as solving some issues around rewrite collisions and WP pages.

It does also potentially collide with some of the behaviour of extended custom post types so we should ensure we account for that. The rewrite rules extended CPTs supports are not currently documented.