Add a call for membership fees email sending system including a management command to send it automatically (1 month before end date of membership, at anniversary date, once a month for 3 months after end date)

Add respect_language utility method to force language use when there is no contexts likes in shell commands

Add non billable possibility to offers. So charge_subscription don't automatically charge corresponding subscriptions. (eg. ILLYSE infra offers)

Signed-off-by: CapsLock <faimaison@legeox.net>