Edit documents as YAML

[YAML](https://yaml.org/) is a nicer language to edit JSON-structured data. I propose a user setting to switch the editor to displaying the documents as YAML.

This YAML would be converted to JSON for saving and converted from JSON for loading by a client-side library. The choice of this library will determine the feature-set (eg forward-references), but obviously JSON-compatible (no custom types or dates, no directives nor productions, tags will be dropped, single-document). Quick search of JS libraries:

* [yaml](https://github.com/eemeli/yaml)
* [js-yaml](https://github.com/nodeca/js-yaml)

I can start a PR when my company accepts the CLA

The issue names no files or tests. Start by locating the document editor and user-settings entry points, then review the proposed YAML and JSON client-side conversion libraries. Done means a user setting switches document display to YAML while saving and loading remain JSON-compatible within the stated limitations.

Written by the indexing model from the issue text.