(core) Exposing custom widgets on the UI

Summary:
Exposing custom widgets as a dropdown menu in custom section configuration panel.

Adding new environmental variable GRIST_WIDGET_LIST_URL that points to a
json file with an array of available widgets. When not present, custom widget menu is
hidden, exposing only Custom URL option.

Available widget list can be fetched from:
https://github.com/gristlabs/grist-widget/releases/download/latest/manifest.json

Test Plan: New tests, and updated old ones.

Reviewers: paulfitz, dsagal

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3127

app/server/lib/TestingHooks.ts

@@ -12,6 +12,7 @@ import {FlexServer} from './FlexServer';
 import {ITestingHooks} from './ITestingHooks';
 import ITestingHooksTI from './ITestingHooks-ti';
 import {connect, fromCallback} from './serverUtils';
+import {WidgetRepositoryImpl} from 'app/server/lib/WidgetRepository';
 
 const tiCheckers = t.createCheckers(ITestingHooksTI, {UserProfile: t.name("object")});
 
@@ -194,4 +195,12 @@ export class TestingHooks implements ITestingHooks {
     }
     return prev;
   }
+
+  public async setWidgetRepositoryUrl(url: string): Promise<void> {
+    const repo = this._server.getWidgetRepository() as WidgetRepositoryImpl;
+    if (!(repo instanceof WidgetRepositoryImpl)) {
+      throw new Error("Unsupported widget repository");
+    }
+    repo.testOverrideUrl(url);
+  }
 }