Write an API for creating and serving attachments

[jrobbins]

We will need a way to upload and view attachments.

An attachment is represented by just a URL string that goes to a URL that is local to our server.

• The API POST method will store the attachment in GCS using the gcs_helpers module. It will return a URL that can be later submitted as part of a form value.
• The serving GET handler will redirect the user to the URL of the attachment in GCS
• For local developers, the serving GET handler will return the actual attachment content