Representative.addresses.phones is reverse foreign key on a reverse
foreign key so that's impossible to use prefetch_related with.

It would need to be denormalized before being usable, let's wait until
someone requires this specific feature.

Contributed by @luxcem in the first place.

remove_countries_without_group and remove_groups_without_mandates are
useful to clean the database before dumping test fixtures

representative_post_save is gone, thanks to memopol rewrite.

It was replaced with representative_pre_import, which allows skipping
some meps.

We're matching on them for group list and detail pages ...

A mixin to encapsulate pre-fetching logic for Representative country and
main_mandate.

It includes latest research by Arnaud Fabre but I ended up squashing all
commits and adding my own fixups in this commit.

Also:

- the import command is tested,
- code was PEP8'ed
- untested code was removed,
- duplicate country Israel was removed and migration was added,
- requirements were deleted in favor of proper setup.py.