Downscale video to save bandwidth and avoid stream-buffering

[stumpner]

Since mediadesk uses the original video for streaming there might be a problem that the stream buffers because of low bandwith.
A new function should downscale videos in different resolutions to save bandwith on low bandwith connections or devices with small screens.

This function is needed also here #10

Infos on ffmpeg see

• https://www.ostechnix.com/20-ffmpeg-commands-beginners/
  Good Forum post of compressing videos
• https://unix.stackexchange.com/questions/28803/how-can-i-reduce-a-videos-size-with-ffmpeg