Split LDAP account presence from Person.status

I would like to change the data model so that the presence of an LDAP account is decoupled from the status in Debian.

This would mean getting rid of "with guest account" statuses (dc_ga, dm_ga) and refactoring the code that needs to be LDAP aware to check, for example, for the existance of a dsa.models.LDAPFields record.

This would make statuses clearer, and also allow to extend LDAPFields with other information, representing for example guest accounts that are present but expired.