Extract image and set as poster frame for video.

There doesn't seem to be a way to have an Elastic Transcoder Preset
that *only* generates thumbnails, so we've created a new Preset based
on the 'Web' System Preset, but we've set the Interval to 99999 (so we
only generate a single image), and Max Width & Max Height to 1280x720
to match the maximum dimensions of the transcoded video.

Adding the `thumbnail_pattern` option to the Output which uses this new
Preset causes the Job to generate the single image that we want. We
then set the `poster` attribute on the `video` element to point at this
image.