Extract ChangeStatus enum to constants.js

Change-Id: I54b36b042a4b76477970ec0a4742655706cc50e8
diff --git a/polygerrit-ui/app/constants/constants.js b/polygerrit-ui/app/constants/constants.js
index 5001dd2..5001ee7 100644
--- a/polygerrit-ui/app/constants/constants.js
+++ b/polygerrit-ui/app/constants/constants.js
@@ -55,3 +55,13 @@
   KEEP_VISIBLE: 'keep-visible',
   NEVER: 'never',
 };
+
+/**
+ * @enum
+ * @desc Specifies status for a change
+ */
+export const ChangeStatus = {
+  ABANDONED: 'ABANDONED',
+  MERGED: 'MERGED',
+  NEW: 'NEW',
+};