Merge pull request #15518 from spray/w/create-akka-http

+pro add akka-http project
fix RequestContext.withXXXHandling methods which fixes two tests
add preliminary HttpEntity.Regular marshaller and better FIXME marshaller exception
!str: #15474: Migrate to reactive-streams 0.4.0.M1
Merge pull request #15578 from drewhk/wip-15474-streams-migrate-spi-0.4-drewhk

Migrate streams to reactive-streams 0.4.M1
Merge remote-tracking branch 'origin/release-2.3-dev' into w/init-http-routing

Conflicts:
	akka-http-core/src/main/scala/akka/http/model/HttpEntity.scala