Delete email addresses from temporary registration keys

When a someone makes a self registration, the email address is saved as a temporary key in a table to prevent the creation of a user with this email address until the registration if confirmed.

If the registration is never confirmed, the email address remains in the table. The user manager is not able to create a user with this email adress.

To sovle this problem:

• Add a timestamp "valid until" to every temporary key.
• Make the duration of the timestamp configurable
  • REST, GUI
  • Registration / Password change
• Implement a job which deletes invalid temporary keys
• If the user manager wants to create a new user with an email address, which is stored as a temporary key, ask him if he wants to cancel the self registration.