This adds support for tracking `m.relates_to`, as well as adding support for the various `/room/{roomID}/relations/...` endpoints to the CS API.
This allows us to apply history visibility without having to recalculate it in the roomserver. Unblocks https://github.com/matrix-org/complement/pull/495, fix missing part of https://github.com/matrix-org/dendrite/issues/617
* Add build page; correct proxy info; fix Caddy example * Improve Caddyfile example * Apply review comments; add polylith Caddyfile