Investigate a less memory-intensive approach for CSV export

Problem to solve

@ahegyi commented in an MR review:

(non-blocker): Just want to point out that the current way we write CSV file seems quite memory heavy:

1. Write data in batches into a tempfile.
2. Load the file into memory.
3. Send it back to the client.

If you would pass a block to the render method, it would yield the tempfile object. With that, you might be able to use the send_file method (https://apidock.com/rails/ActionController/Streaming/send_file).

Proposal

Use send_file to respond to CSV export requests, or alternatively pass the tempfile block to the render method.