(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/gen-server/ApiServer.ts

@@ -12,6 +12,7 @@ import {RequestWithOrg} from 'app/server/lib/extractOrg';
 import * as log from 'app/server/lib/log';
 import {addPermit, getDocScope, getScope, integerParam, isParameterOn, sendOkReply,
         sendReply, stringParam} from 'app/server/lib/requestUtils';
+import {IWidgetRepository} from 'app/server/lib/WidgetRepository';
 import {Request} from 'express';
 
 import {User} from './entity/User';
@@ -98,7 +99,8 @@ export class ApiServer {
    */
   constructor(
     private _app: express.Application,
-    private _dbManager: HomeDBManager
+    private _dbManager: HomeDBManager,
+    private _widgetRepository: IWidgetRepository
   ) {
     this._addEndpoints();
   }
@@ -238,6 +240,13 @@ export class ApiServer {
       return sendReply(req, res, query);
     }));
 
+    // GET /api/widgets/
+    // Get all widget definitions from external source.
+    this._app.get('/api/widgets/', expressWrap(async (req, res) => {
+      const widgetList = await this._widgetRepository.getWidgets();
+      return sendOkReply(req, res, widgetList);
+    }));
+
     // PATCH /api/docs/:did
     // Update the specified doc.
     this._app.patch('/api/docs/:did', expressWrap(async (req, res) => {