Retrieve the default branded link

**This endpoint allows you to retrieve the default branded link.** The default branded link is the actual URL to be used when sending messages. If you have more than one branded link, the default is determined by the following order: * The validated branded link marked as `default` (set when you call the "Create a branded link" endpoint or by calling the "Update a branded link" endpoint on an existing link) * Legacy branded links (migrated from the whitelabel wizard) * Default SendGrid-bran...