(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

package.json

@@ -65,6 +65,8 @@
     "why-is-node-running": "2.0.3"
   },
   "dependencies": {
+    "@googleapis/drive": "0.3.1",
+    "@googleapis/oauth2": "0.2.0",
     "@gristlabs/connect-sqlite3": "0.9.11-grist.1",
     "@gristlabs/express-session": "1.17.0",
     "@gristlabs/pidusage": "2.0.17",
@@ -84,6 +86,7 @@
     "diff-match-patch": "1.0.5",
     "double-ended-queue": "2.1.0-0",
     "electron": "3.0.7",
+    "exceljs": "4.2.1",
     "express": "4.16.4",
     "file-type": "14.1.4",
     "fs-extra": "7.0.0",