* Replace MaterializerSettings with Attributes #25559
* Field access to settings deprecated to make stages use attributes instead
* Internal stages updated to use attributes
* Docs on ActorMaterializerSettings updated to recommend away from using it
* Verify all stages stopped after each testcase in FlowGroupBySpec
* Subscription timeout attributes merged into one
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.
* 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.
