Mysociety-django-popolo

* 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.

* Raise an error on (rather than silenty ignore) inline
areas without an 'id' attribute in the importer.
* Fix the import of areas with parents.
* Don't error if a source or link is missing 'note'.

* Many improvements to the code that imports from Popolo JSON
* Fix the migrations for Python 2 (wfdd)

* Sync with upstream (at 3a34d5cb, plus PR 28).

* Python 3 compatibility changes (James McKinney).