376: Downloads 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/...

Closes #376

Testing

• I added an automated test that confirms the downloadPath has the expected format.
• I was unable to test this manually. This is because this PR depends on the latest changes to the ETL pipeline, which are only deployed to staging, but downloading files from staging is broken Downloads from FMS staging data source fail with 404 #293 (perhaps it has never worked).

Notes

I believe we should merge this despite being unable to manually test it, because we know that the latest ETL pipeline changes will break the current downloadPath logic.

[pgarrison]

@BrianWhitneyAI To-do: review and merge, after #375 is merged

[kmitcham]

Downloads from staging have been problematic; I suspect files uploaded to staging might work for download, but items uploaded to prod I would be surprised if they downloaded from staging