MBL-1338: Re-fetch stored cards after login

Library/ViewModels/PostCampaignCheckoutViewModel.swift

@@ -181,7 +181,7 @@ public class PostCampaignCheckoutViewModel: PostCampaignCheckoutViewModelType,
     // MARK: - Validate Existing Cards
 
     /// Capture current users stored credit cards in the case that we need to validate an existing payment method
-    let storedCardsEvent = initialData.ignoreValues()
+    let storedCardsEvent = Signal.merge(initialData.ignoreValues(), self.userSessionStartedSignal.ignoreValues())
       .switchMap { _ in
         AppEnvironment.current.apiService
           .fetchGraphUser(withStoredCards: true)