S3 Multipart uploads

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Note: This does not yet implement multipart uploading on the server, so you can't upload from GDrive to S3 using Multipart uploads with this PR.

This patch adds some endpoints to the S3 controller for multipart uploads:

• POST /s3/multipart?filename=xyz.jpg Create a new multipart upload. Returns the UploadId and the object key. (The object key can be different from the input filename, if getKey() is defined server side.)
  • [x] (why did I make this a query parameter and not a POST body? TODO change it to a JSON request body)
• GET /s3/multipart/:uploadId List parts that have already been uploaded. Returns an array of { PartNumber, ETag } objects.
  • [x] Currently only supports up to 1000 parts (for a 5GB file), while S3 uploads may have 10000 parts.
• GET /s3/multipart/:uploadId/:partNumber?key=xyz.jpg Get a signed URL to upload a part. Must specify the key that you got in POST /s3/multipart as a query parameter.
• DELETE /s3/multipart/:uploadId?key=xyz.jpg Cancel an upload, discarding parts that were already uploaded. Must specify the key that you got in POST /s3/multipart as a query parameter.
• POST /s3/multipart/:uploadId/complete?key=xyz.jpg Complete an upload. Send a JSON object { parts } in the request body. Each part must have a PartNumber and an ETag. Must specify the key that you got in POST /s3/multipart as a query parameter.

Checkboxes are TODOs.

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Typescript doesn't like the req.uppy references in the S3 controller because the Request type does not declare an uppy property. do you know of a way to deal with that?

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Hmm, CI can't install the dependency from github because git isn't available. I'll try swapping it for a module from npm