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.

app/controllers/admin/backups_controller.rb

@@ -215,7 +215,7 @@ class Admin::BackupsController < Admin::AdminController
   private
 
   def has_enough_space_on_disk?(size)
-    `df -Pk #{Rails.root}/public/backups | awk 'NR==2 {printf "%.0f", $4 * 1024;}'`.to_i > size
+    DiskSpace.free("#{Rails.root}/public/backups") > size
   end
 
   def ensure_backups_enabled