(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"),
 });

app/plugin/PluginManifest.ts

@@ -125,6 +125,13 @@ export interface ImportSource {
    */
   label: string;
 
+  /**
+   * Whether this import source can be exposed on a home screen for all users. Home imports
+   * support only a safeBrowser component and have no access to current document. Primarily used as
+   * an external/cloud storage providers.
+   */
+  safeHome?: boolean;
+
   /**
    * Implementation of ImportSourceAPI. Supports safeBrowser component, which allows you to create
    * custom UI to show to the user. Or describe UI using a .json or .yml config file and use