Multi-select dropdown does not work for multiple locations in Export

[deastman]

Describe the bug
In the VA Export page, the user can select multiple locations for which to download data. When multiple locations are selected, only data for the last location in the list is downloaded.

To Reproduce
Steps to reproduce the behavior:

1. Go to http://0.0.0.0:8000/va_export/
2. Select multiple locations from the Locations searchable dropdown. Be sure that each of these locations have associated VAs
3. Click Download
4. Check the Downloaded file (csv is easiest to check). Note that only the last location entered is present.
5. Download data for each of the Locations individually
6. Note the number of VAs associated with each location

Expected behavior
The downloaded file should include VAs for all locations specified.

Desktop (please complete the following information):

• OS: mac
• Browser: Chrome