DEV: use DiskSpace module for all disk space calculations

This normalizes it so we only carry one place for grabbing disk space size

It also normalizes the command made so it uses Discourse.execute_command
which splits off params in a far cleaner way.

lib/cooked_post_processor.rb

@@ -685,7 +685,7 @@ class CookedPostProcessor
   end
 
   def available_disk_space
-    100 - `df -P #{Rails.root}/public/uploads | tail -1 | tr -s ' ' | cut -d ' ' -f 5`.to_i
+    100 - DiskSpace.percent_free("#{Rails.root}/public/uploads")
   end
 
   def dirty?