Remove hardcoded mp4 limitation (make the container to use a user option, including accepting any)

[GlassedSilver]

Blocked by #579

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Ideally we would always download just what the user wants, e.g. best audio + best video -> merge into whatever container is necessary.

Then for playback transcoding can help if you're on an incompatible client.

This could be avoided by either going with best mp4 as download option or reencoding separately with ffmpeg, but forcing mp4 containers for everyone does have its limits and drawbacks.

Originally posted by @GlassedSilver in #303 (comment)

Relevant error:

ERROR: Postprocessing: file:[FILE PATH AND FILE NAME FOLLOWED BY ".f" and a three digit number like so:].f251.webm: Invalid data found when processing input

[GlassedSilver]

Urgency decreased due to #551 I think. Let's see how this plays out in the mid-run.

[Tzahi12345]

I did try doing this a couple years ago and failed miserably. Lots have changed since then so hopefully it's a tad easier. I do agree this would be a good task for someone to try out and get a feel for the codebase as it touches virtually every part.

I do want to move this to v4.4 as I want to update the latest tag sometime soon.