feat(coderd): add new dispatch logic for coder inbox (#16764)

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.

coderd/notifications.go

@@ -157,6 +157,11 @@ func (api *API) systemNotificationTemplates(rw http.ResponseWriter, r *http.Requ
 func (api *API) notificationDispatchMethods(rw http.ResponseWriter, r *http.Request) {
 	var methods []string
 	for _, nm := range database.AllNotificationMethodValues() {
+		// Skip inbox method as for now this is an implicit delivery target and should not appear
+		// anywhere in the Web UI.
+		if nm == database.NotificationMethodInbox {
+			continue
+		}
 		methods = append(methods, string(nm))
 	}