File upload fields

[jeremydw]

• Add Google Cloud function or App Engine microservice that facilitates server-side URL signing so the client can upload files directly to GCS
• Add JavaScript snippet that shows how to upload files using this endpoint
• Ensure we don't forget CORS support
• A field on the Google Form will record the URL to the uploaded file

[jeremydw]

Implemented as a separate extension for modularity, initial commits are here with working sample file and accompanying backend microservice: https://github.com/grow/grow-ext-file-upload