Fix `/train` endpoint (and potentially other server endpoints) for new data format

With the switch to YAML for 2.0, we forgot to update the /model/train endpoint in the server to be able to handle that, and so the tests are failing/hanging in the PR #6352

It still writes the files to markdown files, that should be yaml now and also the concept of nlu vs stories files doesn’t exist anymore: https://github.com/RasaHQ/rasa/blob/master/rasa/server.py#L746

I haven’t checked if there’s other endpoints that incorrectly handle that, so we may have to address that too

[wochinge]

Is application/json only compatible with markdown at the moment?

Yes! You need to use application/x-yaml for the yaml payload

[tmbo]

sounds great 💯 let me know how things go so I can update the multimedia responses PR