(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/ITestingHooks.ts

@@ -15,4 +15,5 @@ export interface ITestingHooks {
   getDocClientCounts(): Promise<Array<[string, number]>>;
   setActiveDocTimeout(seconds: number): Promise<number>;
   setDiscourseConnectVar(varName: string, value: string|null): Promise<string|null>;
+  setWidgetRepositoryUrl(url: string): Promise<void>;
 }