DEV: Introduce a Service::ActionBase class for service actions

This will help to enforce a consistent pattern for creating service
actions.

This patch also namespaces actions and policies, making everything
related to a service available directly in
`app/services/<concept-name>`, making things more consistent at that
level too.

lib/service/policy_base.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class Service::PolicyBase
+  attr_reader :context
+
+  delegate :guardian, to: :context
+
+  def initialize(context)
+    @context = context
+  end
+
+  def call
+    raise "Not implemented"
+  end
+
+  def reason
+    raise "Not implemented"
+  end
+end