Download data functionality broken on Data Management page

[deastman]

Describe the bug
Clicking the Download Data button on the Data Management page returns a 405.

To Reproduce
Steps to reproduce the behavior:

1. Go to http://0.0.0.0:8000/va_data_management/
2. Click on Download Data button
3. See 405

Expected behavior
Data should download to csv without error.

Screenshots (nice to have)

Desktop (please complete the following information):

• OS: Mac
• Browser: Chrome

Additional context
This functionality was broken by the Export update. Specifically, the Export page uses a POST action, while the Download Data form uses a GET. We don't have tests for the Download Data functionality on the Data Management page, so it was not apparent that this functionality was broken.