reorder imports alphabetically
create user_voicemail service
add user_voicemail association validator
add voicemail_user dao method needed for validation
Fix import in user_voicemail model
implement user_voicemail dao:associate
update user_voicemail find_all_by_user_id to get_by_user_id
Merge remote-tracking branch 'origin/4687_get_user_voicemail_link' into 4688_voicemail_link_2

Conflicts:
	xivo-dao/xivo_dao/data_handler/user_voicemail/dao.py
	xivo-dao/xivo_dao/data_handler/user_voicemail/model.py
	xivo-dao/xivo_dao/data_handler/user_voicemail/services.py
	xivo-dao/xivo_dao/data_handler/user_voicemail/tests/test_dao.py
	xivo-dao/xivo_dao/data_handler/user_voicemail/tests/test_services.py
	xivo-dao/xivo_dao/data_handler/user_voicemail/tests/test_validator.py
	xivo-dao/xivo_dao/data_handler/user_voicemail/validator.py
use user_id instead of voicemail_id for fetching UserVoicemail
add exception for associations that do not exist

Used by RESTAPI when fetching a voicemail associated to a user.
We need to differentiate when a user does not exist from when
the association doesn't exist.
raise error when user does not exist