#6156464 Add Substack Importer UI

Changes proposed in this Pull Request

• Adds Substack Importer UI to Calypso
• Adds a url key to the POST request sent to /importers/new for processing Substack entries from wpcom backend.

Code Changes Overview

1. A new Substack entry in importer-config that contains a new key optionalUrl that contains title, description and invalidDescription and a function validate.
2. A SubstackImporter that continues to use FileImporter.
3. Modifications in UploadingPane to display an optional URL field when the key optionalUrl is present.
4. Client-side validation of URL, including default, error & success states.
5. Modify the startUpload action and wpcom undocumented API to send across a url for /imports/new

Testing instructions

1. Go to Tools > Import and test any existing importer (Blogger for e.g.). Make sure the E2E flow works.
2. Now open Substack. The UI should be self-explanatory. The Upload button should be activated only if a valid file and a valid URL, or a valid file alone is present.
3. The Upload itself will throw an error as we haven’t implemented the backend functionality, but examine the network request to ensure the url is passed correctly:

4. Try to break this & code suggestions welcome! 🙂