Habitat: maximum integration

[themightychris]

DO NOT MERGE

This branch explores integrating habitat to the maximum -- rearranging the /emergence tree completely into the /hab/svc tree. This brings some convenience in deploying to containers as habitat already handles setting up appropriate persistence, but will complicate migrations and perhaps not offer as much convenience as having /emergence be easy to externally mount.

This branch also tried to switch to mariadb, which proved not to be as drop-in as advertised because it diverges in its implementation of JSON columns. slate-cbl is the only known project using JSON columns currently though, and in a very limited fashion, so it might be possible to migrate to mariadb still.

In any case, an initial transition to habitat might better keep mysql and keep the /emergence tree. Deeper moves to habitat might make sense further down the line. This PR bookmarks this work for future review.