ability to remove cookies for IdPDisco

[restena-sw]

If a user selects "Remember IdP" in the built-in discovery service, but selected the wrong one then it's difficult to get rid of the remembered choice (deleting a cookie is hard for some end-users).

After discussion on Slack with @thijskh :

• IdPDisco should only remember the selected IdP if the subsequent auth was successful (reduces risk of saving an immediately-defunct selection)
• expose an API or other callable endpoint that allows an SP to clear the user's cookie. Might also be useful for other cookies besides IdPDisco; i.e. a generic clear-cookie facility

The alternative is to explain to users how to delete cookies, which is cumbersome.

This request originated from a real-life helpdesk issue in a SSP deployment.