(core) Export to Excel and Send to drive

Summary:
Implementing export to excel and send to Google Drive feature.

As part of this feature few things were implemented:
- Server side google authentication exposed on url: (docs, docs-s, or localhost:8080)/auth/google
- Exporting grist documents as an excel file (xlsx)
- Storing exported grist document (in excel format) in Google Drive as a spreadsheet document.

Server side google authentication requires one new environmental variables
- GOOGLE_CLIENT_SECRET (required) used by authentication handler

Test Plan: Browser tests for exporting to excel.

Reviewers: paulfitz, dsagal

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2924

app/gen-server/lib/DocApiForwarder.ts

@@ -42,6 +42,7 @@ export class DocApiForwarder {
     app.use('/api/docs/:docId/remove', withDoc);
     app.delete('/api/docs/:docId', withDoc);
     app.use('/api/docs/:docId/download', withDoc);
+    app.use('/api/docs/:docId/send-to-drive', withDoc);
     app.use('/api/docs/:docId/fork', withDoc);
     app.use('/api/docs/:docId/create-fork', withDoc);
     app.use('/api/docs/:docId/apply', withDoc);