Initialize flipper with default value

- Use the ActiveRecord adapter
 - Use a middle to cache data through memoization (see https://github.com/jnunemaker/flipper/blob/master/docs/Optimization.md)
 - Create the group `admins`: only user which are admins
 - Create `unit_price` feature attached to `admins` group
 - Add method `flipper_id` on User

app/models/spree/user.rb

@@ -135,6 +135,10 @@ module Spree
       spree_orders.incomplete.where(created_by_id: id).order('created_at DESC').first
     end
 
+    def flipper_id
+      "#{self.class.name};#{id}"
+    end
+
     protected
 
     def password_required?

config/initializers/flipper.rb

@@ -0,0 +1,16 @@
+require "flipper"
+require "flipper/adapters/active_record"
+
+Flipper.configure do |config|
+  config.default do
+    Flipper.new(Flipper::Adapters::ActiveRecord.new)
+  end
+end
+Rails.configuration.middleware.use Flipper::Middleware::Memoizer, preload_all: true
+
+Flipper.register(:admins) { |actor| actor.respond_to?(:admin?) && actor.admin? }
+
+if !Flipper[:unit_price].exist?
+  # Unit price default setup, could be overided by admin in the flipper-ui interface
+  Flipper.enable_group :unit_price, :admins
+end