Add a global setting for CDN origin

This is so that, on a multisite cluster, when we handle a CDN request,
the hostname that is requested corresponds to one of the sites -
specifically the default site.

app/models/global_setting.rb

@@ -140,6 +140,7 @@ class GlobalSetting
     hostnames << backup_hostname if backup_hostname.present?
 
     hostnames << URI.parse(cdn_url).host if cdn_url.present?
+    hostnames << cdn_origin_hostname if cdn_origin_hostname.present?
 
     hash["host_names"] = hostnames
     hash["database"] = db_name

config/discourse_defaults.conf

@@ -96,6 +96,9 @@ load_mini_profiler = true
 # recommended, cdn used to access assets
 cdn_url =
 
+# The hostname used by the CDN to request assets
+cdn_origin_hostname =
+
 # comma delimited list of emails that have developer level access
 developer_emails =