User Properties: Cannot add user properties to external users (LDAP)

Provided users (LDAP for example), cannot have user properties because there’s a foreign key constraint in the SAKAI_USER_PROPERTY table referencing a SAKAI_USER record.

Since Sakai can deal with external users, only present in SAKAI_USER_ID_MAP, the foreign key restriction prevents these users to have properties.

In similar cases, like SAKAI_PREFERENCES, this restriction does not exist, both users can have preferences. For consistency reason the foreign key does not make sense.