BE: Seats Currently in Use Members List - export members list

Summary

This is a follow-up iteration to the work completed in #216899 (closed).

In this issue, we will provide users ability to export a list of billable users.

Success Criteria

1. The export should be a filterable file format. (ex. csv - but open to other suggestions)
2. The data presented in the UI, Seat Usage page, should be represented in the export file.
3. If the file cannot be made available immediately, it can also be emailed to the user with a success message in-app at time of request. (Attn - during breakdown if this is needed, please let me know so I can get UX on the success message)

Designs

#282548 (closed)

Availability & Testing

A Controller spec should be added to account for the "export" view/action. The spec should validate that the MIME type is correct (e.g., if we send an CSV file, the MIME should be text/csv and the extension ends in .csv. No need to validate the file contents.