Replace lookup_key with username

Person.lookup_key is an old hack to generate short and user-friendly slug-type identifiers for people on the site.

Unfortunately, its value does not map to anything in the database, and even depends on an external table (like the uid from LDAPInfo when present), making things harder for import/export procedures, and introducing non-standard lookups.

An alternative can be to set that value as Person.username (which would be a supported field in the Django user model), and possibly update it in a housekeeping procedure as the user information changes.