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;
+  }
 };

app/client/ui/MakeCopyMenu.ts

@@ -3,6 +3,7 @@
  * the sample documents (those in the Support user's Examples & Templates workspace).
  */
 
+import {hooks} from 'app/client/Hooks';
 import {makeT} from 'app/client/lib/localization';
 import {AppModel, reportError} from 'app/client/models/AppModel';
 import {DocPageModel} from "app/client/models/DocPageModel";
@@ -302,14 +303,14 @@ export function downloadDocModal(doc: Document, pageModel: DocPageModel) {
       ),
       cssModalButtons(
         dom.domComputed(use =>
-          bigPrimaryButtonLink(`Download`, {
+          bigPrimaryButtonLink(`Download`, hooks.maybeModifyLinkAttrs({
               href: pageModel.appModel.api.getDocAPI(doc.id).getDownloadUrl({
                 template: use(selected) === "template",
                 removeHistory: use(selected) === "nohistory" || use(selected) === "template",
               }),
               target: '_blank',
               download: ''
-            },
+            }),
             dom.on('click', () => {
               ctl.close();
             }),

app/client/ui/ShareMenu.ts

@@ -1,3 +1,4 @@
+import {hooks} from 'app/client/Hooks';
 import {loadUserManager} from 'app/client/lib/imports';
 import {AppModel, reportError} from 'app/client/models/AppModel';
 import {DocInfo, DocPageModel} from 'app/client/models/DocPageModel';
@@ -255,12 +256,12 @@ function menuExports(doc: Document, pageModel: DocPageModel) {
         menuItem(() => downloadDocModal(doc, pageModel),
         menuIcon('Download'), t("Download..."), testId('tb-share-option'))
     ),
-    menuItemLink({ href: gristDoc.getCsvLink(), target: '_blank', download: ''},
+    menuItemLink(hooks.maybeModifyLinkAttrs({ href: gristDoc.getCsvLink(), target: '_blank', download: ''}),
       menuIcon('Download'), t("Export CSV"), testId('tb-share-option')),
-    menuItemLink({
+    menuItemLink(hooks.maybeModifyLinkAttrs({
       href: pageModel.appModel.api.getDocAPI(doc.id).getDownloadXlsxUrl(),
       target: '_blank', download: ''
-    }, menuIcon('Download'), t("Export XLSX"), testId('tb-share-option')),
+    }), menuIcon('Download'), t("Export XLSX"), testId('tb-share-option')),
     (!isFeatureEnabled("sendToDrive") ? null : menuItem(() => sendToDrive(doc, pageModel),
       menuIcon('Download'), t("Send to Google Drive"), testId('tb-share-option'))),
   ];

app/client/ui/ViewLayoutMenu.ts

@@ -1,3 +1,4 @@
+import {hooks} from 'app/client/Hooks';
 import {makeT} from 'app/client/lib/localization';
 import {allCommands} from 'app/client/components/commands';
 import {ViewSectionRec} from 'app/client/models/DocModel';
@@ -76,9 +77,9 @@ export function makeViewLayoutMenu(viewSection: ViewSectionRec, isReadonly: bool
       )
     ),
     menuItemCmd(allCommands.printSection, t("Print widget"), testId('print-section')),
-    menuItemLink({ href: gristDoc.getCsvLink(), target: '_blank', download: ''},
+    menuItemLink(hooks.maybeModifyLinkAttrs({ href: gristDoc.getCsvLink(), target: '_blank', download: ''}),
       t("Download as CSV"), testId('download-section')),
-    menuItemLink({ href: gristDoc.getXlsxActiveViewLink(), target: '_blank', download: ''},
+    menuItemLink(hooks.maybeModifyLinkAttrs({ href: gristDoc.getXlsxActiveViewLink(), target: '_blank', download: ''}),
       t("Download as XLSX"), testId('download-section')),
     dom.maybe((use) => ['detail', 'single'].includes(use(viewSection.parentKey)), () =>
       menuItemCmd(allCommands.editLayout, t("Edit Card Layout"),