Enables having representatives belong to multiple chambers.
Import scripts now rely on the slug to identify representatives.
The remote_id field was moved to a website entity (linking to the
representative page on the chamber website).

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.