Use HTTPS for fetching RSS feeds where possible
On the first fetch:
- If the user enters an HTTP URL, try HTTPS first, then fall back to HTTP
- If the user enters a URL without a scheme, try HTTPS first, then fall back to HTTP
- If the user enters an HTTPS URL, don't fall back to HTTP
- If HTTPS succeeds, save the HTTPS URL
On subsequent fetches, use the scheme from the URL with no fallbacks.