Make response data to github hook plain text or JSON

[txels]

This is relevant In particular for errors.

Internally catching exceptions and changing the output content-type to some specific JSON or text/plain will help debug errors (in the github hook activity log e.g. https://github.com/pretenders/ployst/settings/hooks/3590693). Currently the response is a standard django HTML page and it's painful to read and debug in Github's UI.