Add methods for subscribing event subscribers

The new methods `Spree::Event.unsubscribe_subscribers!` and
`Spree::Event.subscribe_subscribers!` encapsulate the logic
for subscribing and unsubscribing all event subscribers at
once. The logic was previously scattered in engine.rb,
exposing too much of the subscription internals.

core/lib/spree/core/engine.rb

@@ -48,15 +48,11 @@ class Engine < ::Rails::Engine
       initializer 'spree.core.initialize_subscribers' do |app|
         app.reloader.to_prepare do
           Spree::Event.require_subscriber_files
-          Spree::Event.subscribers.each_key do |klass|
-            klass.constantize.subscribe!
-          end
+          Spree::Event.subscribe_subscribers!
         end
 
         app.reloader.before_class_unload do
-          Spree::Event.subscribers.each_key do |klass|
-            klass.constantize.unsubscribe!
-          end
+          Spree::Event.unsubscribe_subscribers!
         end
       end
 

core/lib/spree/event.rb

@@ -136,6 +136,14 @@ def subscribers
       Spree::Config.events.subscribers
     end
 
+    def subscribe_subscribers!
+      subscribers.each_key { |klass| klass.constantize.subscribe! }
+    end
+
+    def unsubscribe_subscribers!
+      subscribers.each_key { |klass| klass.constantize.unsubscribe! }
+    end
+
     private
 
     def normalize_name(name)