Proposal - if payment_processor_type does not define url_site_default then do not present url_site on Processor form

When you go to create a new processor there is a field for the site_url (& some other urls) which I generally don't think it's good to have users fill in and would rather suppress. I think it should be suppressed based on payment processor & while I could make a case for using a supportsUserDefinedUrl method I think we could get away with either

1. don't display the field if payment_processor.site_url_default is empty or
2. don't display the field is payment_processor.site_url_default is 'null' or some other magic value

We aren't currently doing any 'supports' checks on the form & it feels like this is probably something used primarily by older processors so I lean towards 1 being enough

@mattwire @KarinG @adixon @artfulrobot @andrewhunt thoughts