Include JS OpenAPI from app.json in /api responses

[eddyashton]

The endpoints table now stores the openapi elements defined for each endpoint in app.json, and merges these into the resulting document during build_api.

The modules.py test retrieves the OpenAPI docs for the tested JS apps, and checks that they're valid OpenAPI. This required a few minor changes to the existing app.json files.

To support this I've updated the Lua JSON converters, so they can correctly roundtrip an empty object or empty array. This is done by adding a magic field to the metatable on the pushed table. Its an ugly solution, but does what we need for now, while we're still reliant on Lua.

[ghost]

js_dynamic_openapi@15316 aka 20201111.20 vs master ewma over 50 builds from 14780 to 15305

[letmaik]

Why is this necessary?

[eddyashton]

The validator we're using doesn't like this empty object - it doesn't think this matches the schema for a JSON schema object. So we say the same thing ("the items have no further type restriction") by omitting the field entirely.