Server / client notifications

From: https://discuss.tryton.org/t/real-time-notification/224

assigned to @nicoe

added sao tryton trytond type::feature + 1 deleted label

New review60271002 at https://codereview.tryton.org/60271002/#ps1

review60271002 updated at https://codereview.tryton.org/60271002/#ps20001

New review66271002 at https://codereview.tryton.org/66271002/#ps20001

review66271002 updated at https://codereview.tryton.org/66271002/#ps40001

review60271002 updated at https://codereview.tryton.org/60271002/#ps40001

review60271002 updated at https://codereview.tryton.org/60271002/#ps60001

review66271002 updated at https://codereview.tryton.org/66271002/#ps60001

review60271002 updated at https://codereview.tryton.org/60271002/#ps80001

review66271002 updated at https://codereview.tryton.org/66271002/#ps100001

review66271002 updated at https://codereview.tryton.org/66271002/#ps120001

review60271002 updated at https://codereview.tryton.org/60271002/#ps100001

review60271002 updated at https://codereview.tryton.org/60271002/#ps120001

review66271002 updated at https://codereview.tryton.org/66271002/#ps140001

review60271002 updated at https://codereview.tryton.org/60271002/#ps140001

review60271002 updated at https://codereview.tryton.org/60271002/#ps160001

review60271002 updated at https://codereview.tryton.org/60271002/#ps180001

review60271002 updated at https://codereview.tryton.org/60271002/#ps200001

review66271002 updated at https://codereview.tryton.org/66271002/#ps160001