order by completeness

This changes the database code so that the database does not only know if an attributeset is complete, but also how complete it is.

I have several issues with the implementation:

• This duplicates logic from AttributeSet.get_completeness()
• completeness_expr() is defined in filter_queries.py but does not return a Q object
• In order to be able to sort subjects by that value I had to annotate subjects instead of attributesets.