This extends search so it can have consumers that:
1. Can split off "term" from various advanced filters and orders
2. Can build a relation of either order or filter
It also moves a lot of stuff around in the search class for clarity.
Two new APIs are exposed:
`.apply_filter` to apply all the special filters to a posts/topics relation
`.apply_order` to force a particular order (eg: order:latest)
This can then be used by semantic search in Discourse AI