Speed up CSV export

Currently the CSV export can take an unacceptable amount of time to complete when a questionnaire has many questions and 1000s of participants.
It is likely that the export will simply time out in many cases where the questionnaire response data is large.

• Investigate and implement potential performance improvements.
• Include unit tests for comparing:

a) The speed between the old and new method for generating CSV exports
b) The content between the old and new method for generating CSV exports

This does mean that the old code for generating CSVs will need to either:

live on for at least one more version, after which it can be removed

or

be removed, along with the unit tests after test data is acquired