(core) Refactoring google drive plugin

Summary:
Finishing implementation for google drive plugin.
- Refactoring plugin code to make it more robust and to follow grist ux
- Changing the way server hosts untrusted user content, from different domain to different port

Test Plan: Browser tests

Reviewers: dsagal, paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2881

test/nbrowser/testServer.ts

@@ -100,6 +100,7 @@ export class TestServerMerged implements IMochaServer {
       // Set low limits for uploads, for testing.
       GRIST_MAX_UPLOAD_IMPORT_MB: '1',
       GRIST_MAX_UPLOAD_ATTACHMENT_MB: '2',
+      APP_UNTRUSTED_URL : "http://localhost:18096",
       // Run with HOME_PORT, STATIC_PORT, DOC_PORT, DOC_WORKER_COUNT in the environment to override.
       ...(isCore ? {
         HOME_PORT: '8095',