Added displayUsername and username update functionality (#35699)

* Added displayUsername and username update functionality

* fix: move username assignment to safe place

moved the username assignment down a statement so that it doesn't cause exception 

* fix: handle missing username or displayUsername

* refactor: remove redundant code

client/src/redux/fetch-user-saga.js

@@ -20,7 +20,11 @@ function* fetchSessionUser() {
     } = yield call(getSessionUser);
     const appUser = user[result] || {};
     yield put(
-      fetchUserComplete({ user: appUser, username: result, sessionMeta })
+      fetchUserComplete({
+        user: appUser,
+        username: result,
+        sessionMeta
+      })
     );
   } catch (e) {
     yield put(fetchUserError(e));