(core) Adding import from google drive to the home screen

Summary: Importing from google drive from home screen (also for anonymous users)

Test Plan: Browser tests

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2943

app/plugin/PluginManifest-ti.ts

@@ -28,6 +28,7 @@ export const BarePlugin = t.iface([], {
 
 export const ImportSource = t.iface([], {
   "label": "string",
+  "safeHome": t.opt("boolean"),
   "importSource": "Implementation",
   "importProcessor": t.opt("Implementation"),
 });