Fix media processing getting stuck on too much stdin/stderr (#16136)

* Fix media processing getting stuck on too much stdin/stderr

See thoughtbot/terrapin#5

* Remove dependency on paperclip-av-transcoder gem

* Remove dependency on streamio-ffmpeg gem

* Disable stdin on ffmpeg process

lib/paperclip/gif_transcoder.rb

@@ -100,7 +100,8 @@ end
 
 module Paperclip
   # This transcoder is only to be used for the MediaAttachment model
-  # to convert animated gifs to webm
+  # to convert animated GIFs to videos
+
   class GifTranscoder < Paperclip::Processor
     def make
       return File.open(@file.path) unless needs_convert?