Can add shadowed_by_global to a site setting to override

If a site setting has `shadowed_by_global: true` then, if
a `GlobalSetting` exists with the same name then that value
will be used instead.  Additionally, the setting will be hidden.

This is useful in a multisite environment for private settings
such as API keys. You want to set it globally, yet not display
the key to customers.

lib/site_setting_extension.rb

@@ -46,6 +46,10 @@ module SiteSettingExtension
     @choices ||= {}
   end
 
+  def shadowed_settings
+    @shadowed_settings ||= []
+  end
+
   def hidden_settings
     @hidden_settings ||= []
   end
@@ -84,6 +88,15 @@ module SiteSettingExtension
         hidden_settings << name
       end
 
+      # You can "shadow" a site setting with a GlobalSetting. If the GlobalSetting
+      # exists it will be used instead of the setting and the setting will be hidden.
+      # Useful for things like API keys on multisite.
+      if opts[:shadowed_by_global] && GlobalSetting.respond_to?(name)
+        hidden_settings << name
+        shadowed_settings << name
+        current_value = GlobalSetting.send(name)
+      end
+
       if opts[:refresh]
         refresh_settings << name
       end
@@ -97,7 +110,7 @@ module SiteSettingExtension
       end
 
       current[name] = current_value
-      setup_methods(name, current_value)
+      setup_methods(name)
     end
   end
 
@@ -133,7 +146,7 @@ module SiteSettingExtension
   # Retrieve all settings
   def all_settings(include_hidden=false)
     @defaults
-      .reject{|s, _| hidden_settings.include?(s) || include_hidden}
+      .reject{|s, _| hidden_settings.include?(s) && !include_hidden}
       .map do |s, v|
         value = send(s)
         type = types[get_data_type(s, value)]
@@ -177,9 +190,11 @@ module SiteSettingExtension
 
       if deletions.length > 0 || changes.length > 0
         changes.each do |name, val|
+          next if shadowed_settings.include?(name)
           current[name] = val
         end
         deletions.each do |name,val|
+          next if shadowed_settings.include?(name)
           current[name] = defaults[name]
         end
       end
@@ -364,7 +379,7 @@ module SiteSettingExtension
   end
 
 
-  def setup_methods(name, current_value)
+  def setup_methods(name)
     clean_name = name.to_s.sub("?", "")
 
     eval "define_singleton_method :#{clean_name} do