Replacing the uploader / transcoder: integrate Uppy file uploader

[spwoodcock]

The frontend follow on from #188 (comment)

What is Uppy

• Uppy is an open source project by the developers behind the Tus protocol.
• It has direct integration to allow uploading via S3 multipart.
• There is also a plugin called Golden Retriever, to continue uploading after page crash / accidental refresh.
• It also has nice UI elements developed for an uploader dropzone.

Note

S3 multipart should be used.
Tus is nice, but needs an intermediary server to receive data, then upload to S3 on the backend.
Direct multipart upload from the user via pre-signed URLs is more bandwidth efficient.

[linear]

TECH-983