(core) add appsumo endpoints with stub implementations

Summary:
This adds appsumo /token and /notification endpoints, with some
tests.  The stub implementation is sufficient for AppSumo
activation to succeed (when exposed via port forwarding for testing).
It needs fleshing out:

 * Implement upgrade/downgrade/refund and stripe subscription.
 * Implement custom landing page and flow.

Test Plan: added tests

Reviewers: dsagal, georgegevoian

Reviewed By: dsagal

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D2864

app/gen-server/entity/BillingAccount.ts

@@ -12,6 +12,13 @@ interface BillingAccountStatus {
   message?: string;
 }
 
+// A structure for billing options relevant to an external authority, for sites
+// created outside of Grist's regular billing flow.
+export interface ExternalBillingOptions {
+  authority: string;   // The name of the external authority.
+  invoiceId?: string;  // An id of an invoice or other external billing context.
+}
+
 /**
  * This relates organizations to products.  It holds any stripe information
  * needed to be able to update and pay for the product that applies to the
@@ -49,6 +56,12 @@ export class BillingAccount extends BaseEntity {
   @Column({name: 'stripe_plan_id', type: String, nullable: true})
   public stripePlanId: string | null;
 
+  @Column({name: 'external_id', type: String, nullable: true})
+  public externalId: string | null;
+
+  @Column({name: 'external_options', type: nativeValues.jsonEntityType, nullable: true})
+  public externalOptions: ExternalBillingOptions | null;
+
   @OneToMany(type => BillingAccountManager, manager => manager.billingAccount)
   public managers: BillingAccountManager[];