(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/server/lib/FlexServer.ts

@@ -1138,6 +1138,18 @@ export class FlexServer implements GristServer {
     return this.getResourceUrl(doc);
   }
 
+  /**
+   * Get a url for a team site.
+   */
+  public async getOrgUrl(orgKey: string|number): Promise<string> {
+    if (!this.dbManager) { throw new Error('database missing'); }
+    const org = await this.dbManager.getOrg({
+      userId: this.dbManager.getPreviewerUserId(),
+      showAll: true
+    }, orgKey);
+    return this.getResourceUrl(this.dbManager.unwrapQueryResult(org));
+  }
+
   /**
    * Get a url for an organization, workspace, or document.
    */
@@ -1448,7 +1460,7 @@ export class FlexServer implements GristServer {
   private _getBilling(): IBilling {
     if (!this._billing) {
       if (!this.dbManager) { throw new Error("need dbManager"); }
-      this._billing = this.create.Billing(this.dbManager);
+      this._billing = this.create.Billing(this.dbManager, this);
     }
     return this._billing;
   }

app/server/lib/GristServer.ts

@@ -20,6 +20,7 @@ export interface GristServer {
   getHomeUrl(req: express.Request, relPath?: string): string;
   getHomeUrlByDocId(docId: string, relPath?: string): Promise<string>;
   getDocUrl(docId: string): Promise<string>;
+  getOrgUrl(orgKey: string|number): Promise<string>;
   getResourceUrl(resource: Organization|Workspace|Document): Promise<string>;
   getGristConfig(): GristLoadConfig;
   getPermitStore(): IPermitStore;

app/server/lib/ICreate.ts

@@ -17,7 +17,7 @@ import { PluginManager } from 'app/server/lib/PluginManager';
 export interface ICreate {
   LoginSession(comm: Comm, sid: string, domain: string, scopeSession: ScopedSession,
                instanceManager: IInstanceManager|null): ILoginSession;
-  Billing(dbManager: HomeDBManager): IBilling;
+  Billing(dbManager: HomeDBManager, gristConfig: GristServer): IBilling;
   Notifier(dbManager: HomeDBManager, gristConfig: GristServer): INotifier;
   Shell(): IShell|undefined;