Connexion prevents large downloads

[hardbyte]

In /backend/entityservice/serialization.py#L166 we stream the results to avoid having to load the whole dataset into memory at once.

However as in #184, connexion is very strict about json validation. It appears to be consuming the response stream to validate it against the spec.

Possible approaches are:

• change download type to text/csv to avoid validation
• disable validation for the endpoint