Bug: Old data cached in the frontend after external datasets imported

[gdower]

Bug: Old data cached in the frontend after external datasets imported

[mdoering]

I will add a new x-remove-cache-control header to the API response which the backend can use to tell Varnish to remove the Cache-Control headers.
This allows us to cache the response in varnish, but avoid any client caching. I will activate that for external datasets, but not for releases which are most of our load and which are still being allowed to be cached for a week in both varnish and the browser client.

[mdoering]

I have tested this on dev and it works, deploying to prod now