FEATURE: allow specifying s3 config via globals

This refactors handling of s3 so it can be specified via GlobalSetting

This means that in a multisite environment you can configure s3 uploads
without actual sites knowing credentials in s3

It is a critical setting for situations where assets are mirrored to s3.

app/models/backup.rb

@@ -46,7 +46,7 @@ class Backup
 
   def s3
     require "s3_helper" unless defined? S3Helper
-    @s3_helper ||= S3Helper.new(s3_bucket)
+    @s3_helper ||= S3Helper.new(s3_bucket, '', S3Helper.s3_options(SiteSetting))
   end
 
   def upload_to_s3