flatpak: add ffmpeg-full to support video playback

As codec support has changed in Flatpak 19.08, its apps that are supposed to register their interest in them via extension. This is especially important for ARM 32 and AARCH64 as OpenH264 extension provided on the platform level is not available for those architectures. By adding ffmpeg-full, we can restore video playback on many sites.

This will also fix https://bugs.kde.org/show_bug.cgi?id=418030