Use Flipper within the OFN FeatureToggle interface

This ensures that we keep the same interface as before and can migrate
to Flipper in a backwards-compatible way.

lib/open_food_network/feature_toggle.rb

@@ -40,8 +40,12 @@ module OpenFoodNetwork
     end
 
     def enabled?(feature_name, user)
-      feature = features.fetch(feature_name, DefaultFeature.new(feature_name))
-      feature.enabled?(user)
+      if Flipper[feature_name].exist?
+        Flipper.enabled?(feature_name, user)
+      else
+        feature = features.fetch(feature_name, DefaultFeature.new(feature_name))
+        feature.enabled?(user)
+      end
     end
 
     private

spec/lib/open_food_network/feature_toggle_spec.rb

@@ -24,6 +24,17 @@ module OpenFoodNetwork
         expect(FeatureToggle.enabled?(:foo)).to be false
       end
 
+      it "uses Flipper configuration" do
+        Flipper.enable(:foo)
+        expect(FeatureToggle.enabled?(:foo)).to be true
+      end
+
+      it "uses Flipper over static config" do
+        Flipper.enable(:foo, false)
+        stub_foo("true")
+        expect(FeatureToggle.enabled?(:foo)).to be false
+      end
+
       def stub_foo(value)
         allow(ENV).to receive(:fetch).with("OFN_FEATURE_FOO", nil).and_return(value)
       end