add createCustomerPortalSession

src/schema.gql

@@ -1117,6 +1117,9 @@ type Mutation {
   ensureStripeCustomerExists: UserGQL!
   deleteUser: Boolean!
   createSubscription(priceId: String!): CreateSubscriptionResultGQL!
+  createCustomerPortalSession(
+    returnUrl: String = null
+  ): CreateCustomerPortalSessionResultGQL!
   cancelSubscription: UserSubscriptionStatusGQL!
   grantManualEntitlement(
     userId: Int!
@@ -1256,6 +1259,10 @@ type CreateSubscriptionResultGQL {
   sessionId: String!
 }
 
+type CreateCustomerPortalSessionResultGQL {
+  portalUrl: String!
+}
+
 enum CancellationReasonEnum {
   DONT_PLAY_ENOUGH
   TOO_EXPENSIVE