Add FeaturesController for *.js and context for *.jsx

[MAILPOET-2008]

assets/js/src/features/context.jsx

@@ -0,0 +1,4 @@
+import React from 'react';
+import MailPoet from 'mailpoet';
+
+export default React.createContext(MailPoet.FeaturesController);

assets/js/src/features_controller.js

@@ -0,0 +1,7 @@
+const FeaturesController = (config) => ({
+  isSupported: (feature) => {
+    return config[feature] || false;
+  },
+});
+
+export default FeaturesController;

assets/js/src/mailpoet.js

@@ -1,5 +1,9 @@
+import FeaturesController from 'features_controller';
+
 // A placeholder for MailPoet object
-var MailPoet = {};
+var MailPoet = {
+  FeaturesController: FeaturesController(window.mailpoet_feature_flags),
+};
 
 // Expose MailPoet globally
 window.MailPoet = MailPoet;