Move the Side enum to the API

Change-Id: Ie1a7000b66907cd4c2d3001e0325fb5861d3d253
diff --git a/polygerrit-ui/app/constants/constants.ts b/polygerrit-ui/app/constants/constants.ts
index 14e57e4..b0f87d7 100644
--- a/polygerrit-ui/app/constants/constants.ts
+++ b/polygerrit-ui/app/constants/constants.ts
@@ -161,10 +161,7 @@
   HIDDEN = 'HIDDEN',
 }
 
-export enum Side {
-  LEFT = 'left',
-  RIGHT = 'right',
-}
+export {Side} from '../api/diff';
 
 /**
  * The type in ConfigParameterInfo entity.