recurring contrib email links: remove update billing details link when using paypal standard

when you generate a recurring contrib, you receive an email that includes several links for self-processing the recurring profile details. if using paypal standard, the "update billing details" link results in an error. there's a meaningful message – "PayPal Standard processor doesn't support updating subscription billing details." – but we should just suppress the link altogether rather than send people to a link with that message.

I believe we want to handle this in the message template. we already have the $contributeMode method, so I think we can condition on 'notify'