Fix flask_security celery tasks context - fix #1247

[abulte]

patch_flask_security import need to be done after tasks.init_app so that the Celery configuration is taken into account.

#1247 was caused by the config task_serializer being json by default in Celery 4 and our own configuration (set to pickle) not taken into account on flask-security tasks.

[abulte]

NB: it would be even better to use this https://github.com/mattupstate/flask-security/blob/68a556c38c1e247394112774f03e59c96943f5b6/docs/customizing.rst#emails-with-celery instead of patching send_mail but there are other stuff (apart from sending emails delayed) we do in the patched method that I can not see how to reproduce without a patch: theme and i18n logic for template rendering.

[abulte]

About above NB: maybe submit a PR to flask-security with a new hook that would let us define our own way of building the Message object to be sent. This way we could use this new hook and the send_mail_task one and remove the patch.

[noirbizarre]

+1 for the upstream PR to support a custom send_mail_task.
Half of the job is done because now flask-security already use the custom render method in mails too: pallets-eco/flask-security#487

[abulte]

pallets-eco/flask-security#730