Toggle features via env instead of initializer

A FeatureToggle could be switched via a class_eval in an initializer.
The initializer was installed via ofn-install. We want to get rid of
custom, untracked initializers. Here I'm changing the FeatureToggle
class to use environment variables instead.

This change needs to be followed up with a change in ofn-install to use
the new environment variable. It affects only Australian production.

lib/open_food_network/feature_toggle.rb

@@ -1,13 +1,21 @@
 module OpenFoodNetwork
   class FeatureToggle
-    def self.enabled?(feature)
-      !!features.with_indifferent_access[feature]
+    def self.enabled?(feature_name)
+      true?(env_variable_value(feature_name))
     end
 
     private
 
-    def self.features
-      { connect_learn_homepage: false }
+    def self.env_variable_value(feature_name)
+      ENV.fetch(env_variable_name(feature_name), nil)
+    end
+
+    def self.env_variable_name(feature_name)
+      "OFN_FEATURE_#{feature_name.to_s.upcase}"
+    end
+
+    def self.true?(value)
+      value.to_s.casecmp("true").zero?
     end
   end
 end