Allow users to convert .mkv to mp4

Currently when you upload a .mkv file the player does not work as it does not support .mkv file.

In the video card inside project show an option to convert the .mkv to .mp4 or .mov use the ffmpeg the command is this "ffmpeg -i input.mkv -c copy output.mp4"

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* A button like split, but the video goes into processing state.
* The original file will be replaced in R2.