Some providers are refusing the connection when we try to fetch external calendars due missing "User-Agent" header.

Steps to reproduce

1. Get the problematic calendar with support
2. Try to subscribe to it
3. See the error
4. Add the "User-Agent" header
5. See that it works again

Test Notes

• External Calendars still working
• Fetching public calendars in general works
• Problematic URLs from Outlook works
• Fetching Google Calendar public calendars works