Introduce a user preference for sidebar on diff page

This will be used to automatically open the user's preferred sidebar on
diff pages in the UI.

Google-Bug-Id: b/275059901
Release-Notes: Add a user preference for sidebar on the diff page.
Change-Id: Idd8c492aa6cd4cc90bd1b6d581a7cafeb9adee13
diff --git a/polygerrit-ui/app/constants/constants.ts b/polygerrit-ui/app/constants/constants.ts
index d28f33f..24fb5c0 100644
--- a/polygerrit-ui/app/constants/constants.ts
+++ b/polygerrit-ui/app/constants/constants.ts
@@ -263,6 +263,7 @@
     email_strategy: EmailStrategy.ATTENTION_SET_ONLY,
     default_base_for_merges: DefaultBase.AUTO_MERGE,
     allow_browser_notifications: false,
+    diff_page_sidebar: 'NONE',
   };
 }