generic common name field selection (gfk?)

[bennomadic]

Currently, we are using something real dirty: a text-field in the singleton "default certificate values" model. That's dangerous! (i.e, any admin can break havoc by changing it).

To be generic, we should give site admin the freedom to choose any field (and uri pattern).
Using GFK?

caveat: if this is allowed to change, we have to change also the way we're accessing the
webid_url and absolute_webid_uri. I.e, once the certificate has been created, we should always refer to the same URI. I think we should store it as a textfield in the webiduser model (and property acess that, and dynamically building the uri as a fallback).