Change ffmpeg handling

[WithoutPants]

Changes the general behaviour of ffmpeg resolution as follows:

• setup no longer automatically downloads ffmpeg during the initial system creation. Instead, if ffmpeg or ffprobe are not detected, then the user is presented with an option to download ffmpeg as a post-setup step
  
• added settings to set the ffmpeg and ffprobe paths, and added a task to download the binaries (which is done automatically during setup if the user checks the box)
  
• changed the ffmpeg/ffprobe resolution code to check the config directory first, then the path environment, and finally the $HOME/.stash directory - instead of path first, then config, then .stash. This means that the binaries can be overridden without explicitly setting the path by storing them in the config directory.

Resolves #4218