* openapi-mode="follow-acl"(default): follows access control for the
JWT role.
* openapi-mode="ignore-acl": ignores access control for the JWT role.
* openapi-mode="disabled": disables OpenAPI output, the root endpoint
replies with 404 Not Found.
Now that PgVersion is not part of DbStructure, Config is a more apt
module for it.
Also rename getDbStructure to queryDbStructure. AppState also had a
getDbStructure function for a record field.