Use https for explain URL by default

[der-eismann]

Opening the generated HTML reported on a server makes Chrome complain with:

Mixed Content: The page at 'https://example.com/…' was loaded over a secure connection, but contains a form that targets an insecure endpoint 'http://explain.depesz.com/'. This endpoint should be made available over a secure connection.

Since explain.depesz.com works over HTTPS there is no reason not to use it by default.

[darold]

Thanks.