Add a tab for regional assessment downloads

[QSparks]

Create a new tab that allows users to download regional assessments. This tab will feature a region selector and a download button. The files will be hosted on the Nginx file service. We should ensure that any regions without available assessments are omitted from the selection options.

[QSparks]

The first cut looks like this:

And for regions without assessments:

It would be better to restrict region selection to those available. To accomplish this we may need a separate region selector.

TODO:

• Get list of regions that will have assessments.
• Test with large files