Django-popolo

Added
- ElectoralEvent and ElectoralResult classes added.
- personal relationships modelling added.

Changed
- models and tests refactored in order to distribute shortcuts and tests through mixins.
- source added to Identifier.
- tests now encompass on_behalf_of relations.

Added
- Event class added to instances.

Changed
- Main entities primary keys are now numerical IDs.
- django-popolo models aligned to popolo schemas.

Added
- Italian translation in models and admin.

Changed
- Area and inline AreaI18Names admin classes.
- `models.py` code readability increased.

Added
- added tests for importer.

Changed
- `popit` importer substituted by the `popolo_json` importer.
- simplified `popolo_create_from_popit` management task.
- updated travis matrix to latest python (3.6) and django (1.11) releases.

Fixed
- `urls.py` is now compatible with django 1.8>, and does not cause errors in django 1.11.

[Unreleased]: https://gitlab.depp.it/openpolis/django-popolo/compare/v3.0.0...master

* Minimal updates for Django 2 support.

* The importer class can now be used as a mixin.
* Fixes for the truncation of values in related objects on
importing
* The importer management command now uses the generic
truncation option rather than specifying particular fields
to truncate.
* Make the behaviour of the importer when the 'identifiers'
object is missing and [] consistent.
* Add an option to the importer to preserve existing
identifiers with particular schemes.