Cleanup invalid accounts

Suggestion form @pabs:

Accounts should be removed when email is not valid or when it has not been activated for h hours.

Things to consider:

• Using VERP with a random token per user (to avoid crafted bounce at bounce+user@m.d.n that would allow deletion of any user)
• Creating a new plugin called by the worker to cleanup accounts not activated.

I propose to wait 24h before the cleanup.