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.

lib/discourse.rb

@@ -391,7 +391,7 @@ module Discourse
   end
 
   def self.store
-    if SiteSetting.enable_s3_uploads?
+    if SiteSetting.Upload.enable_s3_uploads
       @s3_store_loaded ||= require 'file_store/s3_store'
       FileStore::S3Store.new
     else