Add button to export submissions data as a CSV file

[carlosahs]

The button is added next to the rerun submissions button:

Once clicked, a CSV file will be downloaded. The header of the CSV file contains the following attributes:

attributes = %w[id uploaded_by_id created_at updated_at test_results_count successful_test_results_count language status]

[carlosahs]

Includes ActiveSupport::Testing::TimeHelpers, otherwise it is reported as an uninitialized constant.

[rhomeister]

2 small comments. Ready to go!