Currently the `targets` column in `inbox_notifications` doesn't get
filled. This PR fixes that. Rather than give targets special treatment,
we should put it in the payload like everything else. This correctly
propagates notification targets to the inbox table without much code
change.
This PR is [resolving the dispatch part of Coder
Inbocx](https://github.com/coder/internal/issues/403).
Since the DB layer has been merged - we now want to insert notifications
into Coder Inbox in parallel of the other delivery target.
To do so, we push two messages instead of one using the `Enqueue`
method.
