Add used feature toggles to appear in the admin UI

The admin UI shows only features which were added to the database. We
always had to look up a feature's name add it manually before we could
enable it. Now it will appear in the UI after some code asked if it was
enabled.

This only adds a database query once when the feature doesn't exist yet.
The `feature.exist?` call only performs an in-memory lookup in the
memoizer.

lib/open_food_network/feature_toggle.rb

@@ -3,15 +3,14 @@
 module OpenFoodNetwork
   # Feature toggles are configured via Flipper.
   #
-  # We define features in the initializer and then it can be customised via the
-  # web interface on each server.
-  #
   # - config/initializers/flipper.rb
   # - http://localhost:3000/admin/feature-toggle/features
   #
   module FeatureToggle
     def self.enabled?(feature_name, user = nil)
-      Flipper.enabled?(feature_name, user)
+      feature = Flipper.feature(feature_name)
+      feature.add unless feature.exist?
+      feature.enabled?(user)
     end
   end
 end