Introduces a materializer started through an extension and then an implicit
conversion for Scala turning an implicitly available ActorSystem into a
materializer. The Java APIs has been ammended with run-methods accepting
an ActorSystem.
* Adds internal access to materializer before initialization (#25324)
* Implements new actorRef source based on graph stage (#25324)
* Removes obsolete actorRef source (#25324)
* Improves backwards compatibility with old implementation (#25324)
* Removes dedicated new subclass for materializer access again (#25324)
* Improves implementation (#25324)
* Finalizes implementation (#25324)
* Small improvements to API and documentation (#25324)
* Completion strategy as a replacement for poison pill (#25324)
* Adding more tests and updating the documentation (#25324)
* ⇒, →, ←
* because we don't want to show them in documentation snippets and
then it's complicated to avoid that when snippets are
located in src/test/scala in individual modules
* dont replace object `→` in FSM.scala and PersistentFSM.scala
* Add CopyrightHeader support for sbt-boilerplate plugin.
* Add CopyrightHeader support for `*.proto` files.
* Add regex match for both `–` and `-` for CopyrightHeader.
* Add CopyrightHeader support for sbt build files.
* Update copyright from 2018 to 2019.
