How to deal with deleted payment and subscription sources and the source URLs?

[remcotolsma]

We don't intentionally add dead links, we just link, there is a difference? A 404 message or something similar. is not necessarily a big deal, then it is also clear to users that the source has been removed. Shall we restore the situation as before pronamic/wp-pronamic-pay-gravityforms@fe6438a? This is simple in terms of code, performance and still gives users a quick link to the source. In a new issue we can then investigate whether we should check the existence of the source?

Originally posted by @remcotolsma in pronamic/wp-pronamic-pay-gravityforms#39 (comment)