!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
Btw. we need to make sure to keep syntax changes like these at an absolute minimum. Those pure syntax changes are an enormous hassle for everyone who needs to maintain long-running branches as a change like that will introduce spurious merge conflicts that often need to be dealt with manually (and often repeatedly).