Aureplop/clean channels

General

• Relative packages to websockets (channels, asgiref, daphne, asgi-redis) are updated to their last official release.
• channels doesn't come from a fork anymore. Instead we define new DjangoJsonWebsocketConsumer, which subclasses JsonWebsocketConsumer from channels, and which uses DjangoJSONEncoder to encode message as json.
• Defines PermConsumerMixin which handles permission checks on connection to a consumer.

KPsul websocket

• Check if connecting users to this consumer have the kfet.is_team permission.
• Clean code of useless elements in this consumer.

Fixes #67.