Fix self-service recurring link on dashboard

Fix self-service recurring link on dashboard

This picks up on two of the issues that were blocking on
https://github.com/civicrm/civicrm-core/pull/18787 and
https://github.com/civicrm/civicrm-core/pull/18196
(but does not solve all of them). It

1) adds the basis for unit test cover and
2) separates the function for retrieving links
for self-service from back office user.

I think there is still some jumbling together of these concepts but
as long as we are careful to extend the tests as we extend the logic...

This fixes what I suspect is an oldish regression whereby the cancel link
for paypal std users was wrong