Validate uploaded blob sizes

[bcspragu]

Saw this in #78, filing for posterity.

We use presigned Azure blob URLs to allow end-users to upload files. Unfortunately, I don't think Azure allows limiting upload size for URLs [1] [2], so we'll want to check the size of the blob when the user says the upload is done.

[1] https://learn.microsoft.com/en-us/rest/api/storageservices/create-user-delegation-sas
[2] https://stackoverflow.com/questions/50751853/limit-upload-size-of-a-blob-to-azure-blob-storage