WebM support on iOS

At the moment, the mobile versions of discord for iOS and iPadOS can't play WebM files.

WebM is a video container that uses a VP8 or VP9 video channel and vorbis or opus audio channel. Discord already uses opus for voice calls, and VP9 is supported in iOS14.
And while VP8 and Vorbis don't have OS support, third party apps can code in support for it.

It should be easy to make it work.

I don't own an iPhone myself, but it's annoying when i send a webm to one of my friends and they can't see it.