Transcode video using AWS Elastic Transcoder.

* Transcode video to MP4 format using the 'Web' system preset
* Store result in 'output' sub-directory within input S3 directory
* Assume output is available immediately after job is submitted

Notes:

* We've hard-coded the first part of the output URL for now, but this
is a bit ugly, because it involves some duplication.
* In reality output is not available immediately and we ought to either
poll the job to see if it has completed or use SNS to get a
notification when the job completes.