!str #15551 Switch to implicit passing of FlowMaterializer

* implicit param
* change tests to use implicit materializer
* add ActorFlow trait that provides an implicit materializer inside an actor
  in the right way, i.e. encourage usage of that instead of
  implicit def mat(implicit arf: ActorRef): FlowMaterializer
* make http compile, but those (@jrudolph, @sirthias) who know the api better will have to adjust
  to take full advantage of the implicit materializer

Refs #15551
This approach is safe after #15701
Of course. There is at least one being planned, though, and there’s nothing we can do about it. One way to keep the hassle small is to not have long-running branches off of release-2.3-dev; there should not really be a need to in any case, do you have a specific use-case?