Create management command to fill in duration values for audio and video files

[rtibbles]

Target: Hotfixes

• Add ffmpeg back to the Dockerfile dependencies
• Create a management command that:
  • Loops through all file objects that have an audio or video preset and for which duration is null
  • Gets that file from GCS
  • Reads the duration from it using ffprobe (example command here: https://github.com/learningequality/ricecooker/blob/develop/ricecooker/utils/videos.py#L107)
  • Sets that duration on all files that share the same MD5

[rtibbles]

Fixed in #3408