(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/server/mergedServerMain.ts

@@ -121,6 +121,7 @@ export async function main(port: number, serverTypes: ServerType[],
     server.addBillingPages();
     server.addWelcomePaths();
     server.addLogEndpoint();
+    server.addGoogleAuthEndpoint();
   }
 
   if (includeDocs) {