storage managers: add mirror storage manager.

Add a storage manager that allows mirroring files to multiple storage
backends.

app/logical/storage_manager/mirror.rb

@@ -0,0 +1,29 @@
+# A StorageManager that mirrors files across multiple storage backends.
+class StorageManager::Mirror < StorageManager
+  attr_reader :services
+
+  def initialize(services, **options)
+    @services = services
+    super(**options)
+  end
+
+  def store(io, dest_path)
+    services.each do |service|
+      service.store(io, dest_path)
+    end
+  end
+
+  def delete(path)
+    services.each do |service|
+      service.delete(path)
+    end
+  end
+
+  def open(path)
+    services.first.open(path)
+  end
+
+  def file_url(path)
+    services.first.file_url(path)
+  end
+end