Support for Apple tvOS

Originally reported https://github.com/slackhq/nebula/issues/1546

This is actually more complicated than I initially imagined, turns out flutter doesn't support tvOS yet so while our backend would likely work just fine there is no good answer for the frontend in this architecture.

Main flutter issue: https://github.com/flutter/flutter/issues/47928

Hacky stuff that kinda might work:

• https://github.com/LibertyGlobal/flutter-tvos-demo
• https://github.com/DenisovAV/flutter_tv
• medium.com/flutter-community/flutter-for-apple-tv-756fcd5e8113