(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/common/UserAPI.ts

@@ -6,6 +6,7 @@ import {BrowserSettings} from 'app/common/BrowserSettings';
 import {BulkColValues, TableColValues, UserAction} from 'app/common/DocActions';
 import {DocCreationInfo, OpenDocMode} from 'app/common/DocListAPI';
 import {Features} from 'app/common/Features';
+import {ICustomWidget} from 'app/common/CustomWidget';
 import {isClient} from 'app/common/gristUrls';
 import {FullUser} from 'app/common/LoginSessionAPI';
 import {OrgPrefs, UserOrgPrefs, UserPrefs} from 'app/common/Prefs';
@@ -321,6 +322,7 @@ export interface UserAPI {
   deleteUser(userId: number, name: string): Promise<void>;
   getBaseUrl(): string;  // Get the prefix for all the endpoints this object wraps.
   forRemoved(): UserAPI; // Get a version of the API that works on removed resources.
+  getWidgets(): Promise<ICustomWidget[]>;
 }
 
 /**
@@ -428,6 +430,10 @@ export class UserAPIImpl extends BaseAPI implements UserAPI {
     return this.requestJson(`${this._url}/api/templates?onlyFeatured=${onlyFeatured ? 1 : 0}`, { method: 'GET' });
   }
 
+  public async getWidgets(): Promise<ICustomWidget[]> {
+    return await this.requestJson(`${this._url}/api/widgets`, { method: 'GET' });
+  }
+
   public async getDoc(docId: string): Promise<Document> {
     return this.requestJson(`${this._url}/api/docs/${docId}`, { method: 'GET' });
   }