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/client/DefaultHooks.ts

@@ -1,11 +1,22 @@
 import { UrlTweaks } from 'app/common/gristUrls';
+import { IAttrObj } from 'grainjs';
 
 export interface IHooks {
   iframeAttributes?: Record<string, any>,
   fetch?: typeof fetch,
   baseURI?: string,
   urlTweaks?: UrlTweaks,
+
+  /**
+   * Modify the attributes of an <a> dom element.
+   * Convenient in grist-static to directly hook up a
+   * download link with the function that provides the data.
+   */
+  maybeModifyLinkAttrs(attrs: IAttrObj): IAttrObj;
 }
 
 export const defaultHooks: IHooks = {
+  maybeModifyLinkAttrs(attrs: IAttrObj) {
+    return attrs;
+  }
 };