Serve JSON Schema with MIME media type when it gets formalised

[QuintinWillison]

For the time being it's only 'proposed' - see draft-handrews-json-schema-02, Section 13 - but it's likely to be something like application/schema+json.

Currently our S3 host is responding with Content-Type of application/json as you might expect. e.g. with:

curl -I https://schemas.ably.com/json/app-stats-0.0.1.json

Blocked by json-schema-org/json-schema-spec#680