Use the `exception` log method for better exception logging. This change the log
level to 'ERROR'

All import processes now use a general try-catch during the import of each data
item. This ensures that even in the event of an error (due to a malformed data
piece for example), the import process will continue.

Errors caught this way are logged at the debug level.

Needed because names are not unique (already 2 cases of 2 meps with the same name)

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_post_save is gone, thanks to memopol rewrite.

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

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.