Flask-rest-jsonapi-next

- fix: API blueprint should respech `Flask.config.APPLICATION_ROOT`

- fix: more problems with content negotiation: API was allowing different `Accept`
header, but was still forcing `Content-Type: application/vnd.api+json` in responses

- fix: marshmallow3 compatibility - `fields.List.container` attribute no longer exists

- fix: API returns 404 on `/resource/<int:id>` routes when no object is found

- feat: allows "multipart/form-data" content type on all POST and PATCH requests
- this is not ideal, but we need it to support file uploads for some of our APIs
- in the future, proper content type negotiation must be implemented

- feat: while app exception handling and JSON:API error responses
- feat: use all decorators from all Resource subclasses
- feat: POST and PATCH schemas can be separately declared from `Resource.schema`
- optional call to `model.validate()` in data layer
- support for CSV in simple filter URL parameters (`?filter[foo]=1,2,3`)
- use `rapidjson` if it is installed