Fixed:
- Classification code and descr fields can now be null, in order to use this class for tagging
- constituency_descr_tmp and electoral_list_descr_tmp moved from Area to Membership
- role fields max_length in Membership and Post objects increased to 512
- get_former_parents and get_former_children moment_date parameter can be null
- get_former_parents and get_former_children FIP classification_type corrected