(core) correct capitalization of AppSumo; add account management link

Summary:
Branding feedback from AppSumo found a capitalization problem. They also nudged us again to include a link back for the user to manage
their AppSumo account.

Test Plan: manual

Reviewers: dsagal, anaisconce

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D2901

app/client/ui/BillingPage.ts

@@ -182,6 +182,7 @@ export class BillingPage extends Disposable {
             makeSummaryFeature([`Your next invoice is `, getPriceString(sub.nextTotal),
                                 ' on ', dateFmt(sub.periodEnd)]),
           ] : null,
+          tier ? this.buildAppSumoLink(this._appModel.currentOrg?.billingAccount?.externalOptions?.invoiceId) : null,
           getSubscriptionProblem(sub),
           testId('summary')
         ),
@@ -250,6 +251,22 @@ export class BillingPage extends Disposable {
     return null;
   }
 
+  // Include a precise link back to AppSumo for changing plans.
+  public buildAppSumoLink(invoiceId: string | undefined) {
+    if (!invoiceId) { return null; }
+    return dom('div',
+               css.billingTextBtn({ style: 'margin: 10px 0;' },
+                                  cssBreadcrumbsLink(
+                                    css.billingIcon('Plus'), 'Change your AppSumo plan',
+                                    {
+                                      href: `https://appsumo.com/account/redemption/${invoiceId}/#change-plan`,
+                                      target: '_blank'
+                                    },
+                                    testId('appsumo-link')
+                                  )
+                                 ));
+  }
+
   public buildPlansPage() {
     // Fetch plan and card data if not already present.
     this._model.fetchData().catch(reportError);

app/common/UserAPI.ts

@@ -65,6 +65,9 @@ export interface BillingAccount {
   individual: boolean;
   product: Product;
   isManager: boolean;
+  externalOptions?: {
+    invoiceId?: string;
+  };
 }
 
 // Information about the product associated with an org or orgs.