Set proper metadata in transcode task output MP4

Nobody has claimed this yet.

The output task is currently creating a video file that has no major_brand whatsoever in the MP4 container.
It also misses a lot of other metadata in the file, and browsers can't even play it back (they just prompt for
a download instead).

This is also a problem because the only way of detecting .mp4 vs .mov files is by checking the `major_brand`
of the `ffprobe` output.

[Here](https://livepeercdn.com/asset/216fnrl21vng3ney/video.json) is an example of such `ffprobe` output
not including a major brand, which was run on an asset transcoded by our own task. This is the problematic part:
```json
"tags": {
**"major_brand": "",**
"minor_version": "",
"compatible_brands": "",
"creation_time": "2020-01-10T19:37:32.000000Z"
}
```

This kind of blocks #17 since we can't start depending on a field which we generate broken values ourselves.

Start at the transcode task that produces the MP4 and compare its output with the linked ffprobe JSON, focusing on the missing major_brand and related metadata. Confirm completion by producing an MP4 whose ffprobe tags are populated and that browsers play rather than download it.

Written by the indexing model from the issue text.