Add new `Delivery.uploader` property

[agrathwohl]

Upon completion of https://github.com/Storyboard-fm/storyboard-serverless/issues/131 we will be able to pass metadata obtained from the Uppy tus plugin to our serverless lambda functions.

In order to store the uploader in our dynamodb tables, we'll:

• create a new property, Delivery.uploader, which provides the UUID of the account responsible for the upload event.
• Ensure Source and Asset instances inside the Delivery inherit this property into their objects as well.