Feed Fedora Discussion user notifications into the new notifications system

[Zlopez]

Goal: integrate Fedora Discussion notifications into the new overall notification system

Implementation:

Discourse can fire a webhook whenever a notification is generated. We have an existing webhook that sends some information to fedora-messaging, but not these, since they are personal. So, we'd either need the FMN replacement to accept these webhooks directly, or possibly to deliver them via the non-public message bus.

However, that's an insider detail. Users should just see them as another source of notifications, and be able to configure the output as they like.

This was originally opened on fedora-infrastructure tracker, but it is more related to FMN.

There were two possible solution that we came up for this on CPE F2F:

1. Create private rabbitmq queue, that will be consumed by FMN, which will send it forward
2. FMN will listen directly to discourse events and just forward these messages

[amoloney1]

This request is out of scope for the FMN team project at the time of opening, however once the new service is in use for several months, this request can be scoped and planned accordingly.