Keep track of creation date for unused accounts

[babs]

As an admin, I'd like to be able to cleanup unused accounts (inc. never used accounts).

Currently, it's doable via LastUpdate but only if the account has been used at least once.

This issue is resolved in #321

[maddes-b]

I also thought about this, as this would help to delete dead registrations after a year or so.
They could still re-register if needed again.

[babs]

Exactly the intended purpose :)