Auto-create Person on User creation

If a User is created, AlekSIS should auto-create a Person object (or try to match one). This must be configurable via constance.

The goal is to have three options:

• Do nothing
• Auto-create a Person, duplicating the data fields that are available in the User object
• Try to match an existing Person via a selectable set of fields (i.e. full name, e-mail address)

The practical main goal here is that people logging in through LDAP get a user account created, and immediately also get a Person associated.

Sub-task of AlekSIS#47.