Migrate from ElasticTranscoder to MediaConvert

[mbertrand]

Amazon's MediaConvert service is cheaper than ElasticTranscoder and supports more features/formats. We should migrate to that, preferably via a common ol-django app that can be shared by both OVS and OCW-Studio.

Designs and Mockups

Using the MediaConvert code in OCW Studio as a starting point, move as much common functionality as possible into a new ol-django app. Refactor OVS (and OCW Studio) to use this app instead (preset output format JSON, job submission, transcode success/failure webhook handling, etc).

Acceptance Criteria:

• All current functionality and video output formats remain the same, but with MediaConvert handling the transcode jobs.