Enhance soundposts

[AlexZorzi]

it would be cool if there was a way to also download soundposts as webm's with sound included, instead of only as no audio files. so they are easier to share outside of imageboards.

the support for soundposts was originally added at ab58e9d

[Adamantcheese]

This would require muxing, something the Android APIs don't do particularly well from back when I was taking a look at this and I don't want to pull in ffmpeg as a binary dependency because it's massive.

[AlexZorzi]

I tried https://github.com/Docile-Alligator/Infinity-For-Reddit, reddit has the same problem of audio and video being separate but its capable of stitching together videos and audio without problem