Part of https://github.com/matrix-org/matrix-public-archive/issues/6
Use JSON5 for configuration files with comments. Now we can leave the available config in `config.default.json` without having to add weird instructions to remove the `xxx`, etc - https://www.npmjs.com/package/json5 - https://www.npmjs.com/package/nconf - https://github.com/indexzero/nconf/issues/113#issuecomment-69999413