(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/sendAppPage.ts

@@ -45,6 +45,7 @@ export function makeGristConfig(homeUrl: string|null, extra: Partial<GristLoadCo
     maxUploadSizeImport: (Number(process.env.GRIST_MAX_UPLOAD_IMPORT_MB) * 1024 * 1024) || undefined,
     maxUploadSizeAttachment: (Number(process.env.GRIST_MAX_UPLOAD_ATTACHMENT_MB) * 1024 * 1024) || undefined,
     timestampMs: Date.now(),
+    enableWidgetRepository: Boolean(process.env.GRIST_WIDGET_LIST_URL),
     ...extra,
   };
 }