* Add "Welcome" type to Broadcasts, allow dynamic "authoring" of Broadcasts
This adds a new `type_of` to the Broadcast model: "Welcome". As we begin to create a new "welcome notification" workflow, we'll categorize them by making them all of the same type.
This also adds a concept of a "welcoming user", which can be set via an ENV var. The WELCOMING_USER_ID will allow us to explicitly set (and change) which user in the database is the one that "sends" welcoming notification. In production, we plan for this to be dev.to/sloan.
* Allow WelcomeNotificationWorker to accept + send any kind of broadcast
This abstracts out the logic of deciding which broadcast to send from the woker into the calling method. This will help us send many different kinds of welcome notifications using one, resuable worker class.
* Raise if a Notification can't be created
* Add STAFF_USER_ID as an ENV var
This is in preparation for moving away from SiteConfig.staff_user_id, and replacing it with an ENV var. We will need to set this in production first, and then make a separate PR to replace all instances of staff_user_id with the newly-set ENV var.
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