add hooks for tweaking how downloads happen (for grist-static) (#665)

This makes three main changes:
  * Adds a hook to transform download links.
  * Adds a hook to add an externally created ActiveDoc to a DocManager.
  * Rejiggers XLSX export code so it can be used without streaming,
    which is currently tricky in a browser. Regular usage with node
    continues to use streaming.

With these changes, I have a POC in hand that updates grist-static
to support downloading CSVs, XLSXs, and .grist files.

app/server/lib/DocManager.ts

@@ -133,6 +133,14 @@ export class DocManager extends EventEmitter {
     return this.createNamedDoc(docSession, 'Untitled');
   }
 
+  /**
+   * Add an ActiveDoc created externally. This is a hook used by
+   * grist-static.
+   */
+  public addActiveDoc(docId: string, activeDoc: ActiveDoc) {
+    this._activeDocs.set(docId, Promise.resolve(activeDoc));
+  }
+
   public async createNamedDoc(docSession: OptDocSession, docId: string): Promise<string> {
     const activeDoc: ActiveDoc = await this.createNewEmptyDoc(docSession, docId);
     await activeDoc.addInitialTable(docSession);