(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/migration/1623871765992-ExternalBilling.ts

@@ -0,0 +1,24 @@
+import { nativeValues } from 'app/gen-server/lib/values';
+import { MigrationInterface, QueryRunner, TableColumn } from 'typeorm';
+
+export class ExternalBilling1623871765992 implements MigrationInterface {
+
+  public async up(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.addColumn('billing_accounts', new TableColumn({
+      name: 'external_id',
+      type: 'varchar',
+      isNullable: true,
+      isUnique: true,
+    }));
+    await queryRunner.addColumn('billing_accounts', new TableColumn({
+      name: 'external_options',
+      type: nativeValues.jsonType,
+      isNullable: true,
+    }));
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.dropColumn('billing_accounts', 'external_id');
+    await queryRunner.dropColumn('billing_accounts', 'external_options');
+  }
+}