Downloads should use labkey URL when file is available on Vast

[pgarrison]

Cloud-first FMS backend changes and existing updates for them (#370, #375) cause downloads to use the S3 URL. But for files available on the Vast, the download will be faster and use fewer resources if we download from the Vast via the labkey URL http://aics.corp.alleninstitute.org/labkey/fmsfiles/image/allen/...