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
