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/attachment_extensions.rb

@@ -2,6 +2,10 @@
 
 module Paperclip
   module AttachmentExtensions
+    def meta
+      instance_read(:meta)
+    end
+
     # We overwrite this method to support delayed processing in
     # Sidekiq. Since we process the original file to reduce disk
     # usage, and we still want to generate thumbnails straight