rename 'featureToggle' to 'isFeatureEnable' and move definition of available prerelease-features to constants.jsx

web/react/utils/constants.jsx

@@ -399,5 +399,11 @@ export default {
     NotificationPrefs: {
         MENTION: 'mention'
     },
-    FeatureTogglePrefix: 'feature_enabled_'
+    FeatureTogglePrefix: 'feature_enabled_',
+    PRE_RELEASE_FEATURES: {
+        MARKDOWN_PREVIEW: {
+            label: 'markdown_preview', // github issue: https://github.com/mattermost/platform/pull/1389
+            description: 'Show markdown preview option in message input box'
+        }
+    }
 };