Export files to USB in consistent directory structure

[adghayes]

On creation of this issue, we export files to a bunch of different places on USB drives:

• VxAdmin: ballot packages are exported to /ballot-packages
• VxScan + VxCentralScan: cast vote records are exported to /cast-vote-records/election-identifier
• VxAdmin: many documents are just exported to /

We've decided that, where possible, we want to stick to a consistent structure in the format: /election-identifier/[file-type]/[filename]. This allows easy copying and deletion of most files for an election. Most importantly, it provides a consistent UX.

The following places need to use the new format:

• VxScan + VxCentralScan - cast vote record exports
• VxScan + VxCentralScan - backups?
• VxAdmin - ballot package export (assuming this continues to exist)
• VxAdmin - .csv exports
• VxAdmin - .pdf exports