Return national vermdata file when requesting vermdata for null district

[Brian Lewis]

This is a convenience when a district is being selected in a dashboard - choosing all districts will result in selecting the national file.

Also, return 404 when the requested file cannot be found.

All vermdata files are now assumed to include the context in their title; e.g.

vermdata_fedemis_chk

vermdata_kemis

etc

[Brian Lewis]

Pull Request #280 closes