models.Contactable: add basic_name

The `basic_name' field is automatically updated when the model is
saved.  It contains a text which can be used to sort all
Contactable entried regardless of their types.
models.Contactable: add `contacts' property

This property returns all the contacts and also
searches in members if no contacts are provided
for a Person or Organisation entry.
models.Member: use Person.name_nn