Convert layers to YAML

Closes #711

Although requiring a script is not ideal, the JSON MML file couldn't be sanely edited anyways. The advantages are

* Sane merge conflict resolution
* Not always a merge conflict for edits to the same SQL
* Reasonable code reviews of SQL changes. Currently I don't review most SQL changes since they're very hard to get a handle on when escaped into a single line of JSON
* Easier changing of database settings. With YAML aliases, they only need to be specified once

I think it'd be easiest to merge this then rebase other merges on top of it - the one-time conversion from JSON to YAML is not trivial, as a great deal of work had to be done by hand to clean up the YAML.
As you dropped the quotes in the citywalls layer, you can remove them elsewhere as well.