MM-21481: Fixed several issues from user marshalling (#13627)

* MM-21481: Fixed several issues from user marshalling

- Fixed the root cause for panic by properly converting
the map to a User struct.
- Added a check for type conversion for extra safety.
- Fixed a somewhat unrelated issue of a pointer to pointer reference.

* Fix tests

api4/user_test.go

@@ -1687,11 +1687,9 @@ func assertExpectedWebsocketEvent(t *testing.T, client *model.WebSocketClient, e
 
 func assertWebsocketEventUserUpdatedWithEmail(t *testing.T, client *model.WebSocketClient, email string) {
 	assertExpectedWebsocketEvent(t, client, model.WEBSOCKET_EVENT_USER_UPDATED, func(event *model.WebSocketEvent) {
-		eventUser, ok := event.GetData()["user"].(map[string]interface{})
+		eventUser, ok := event.GetData()["user"].(*model.User)
 		require.True(t, ok, "expected user")
-		userEmail, ok := eventUser["email"].(string)
-		require.Truef(t, ok, "expected email %s, but got nil", email)
-		assert.Equal(t, email, userEmail)
+		assert.Equal(t, email, eventUser.Email)
 	})
 }