refactor: Add CsvRecord class instead of using adhoc CsvRecord struct

[frankchn]

Problem

Currently, the decryption worker returns an ad-hoc CsvRecord struct for consumption that is constructed in a piecemeal basis in the business logic.

Solution

We unify the logic to generate the CsvRecord struct in a class with the appropriate methods.

Note that we cannot post a full-fledged object (i.e. including JS methods on the object) using postMessage, so we construct the appropriate fields as properties of the original CsvRecord, so minimal modification to submission.client.factory.js is necessary.

Tests

Ensure that CSV and attachment downloads continue to work as expected.

[liangyuanruo]

I'd like to suggest converting updateSubmissionData to become a getter method instead - please lmk your thoughts!