Export play-grpc to the play org

[dwijnand]

This code has been exported out of the akka-grpc repository into
https://github.com/playframework/play-grpc.

The last release from this repo was:

com.lightbend.akka.grpc : akka-grpc-play-testkit : 0.4.2

This is now available from the play-grpc repo as:

com.lightbend.play : play-grpc-testkit : 0.5.0-M5

(Still need to sync it from JCenter to Maven Central.)

... and I just realised that I need to change the package from
akka.grpc.play to something else... Will do!

Tracked as playframework/play-grpc#12

Fixes #362
Fixes #300

[dwijnand]

Need to fix the docs:

Error writing content for page play-framework.html: Unknown snippet [/localhome/jenkinsakka/workspace/akka-grpc-pr-validator/play-interop-test-scala/src/main/protobuf/helloworld.proto (No such file or directory)] referenced from [play-framework.html]

[dwijnand]

Hmm, @raboof should the play interop projects live here or in play-grpc?

[raboof]

@dwijnand I think it makes sense to have them in play-grpc, WDYT?

[dwijnand]

@raboof are PlayScalaClientCodeGenerator and friends staying here?

My goal is trying to figure out how to land this PR ASAP, and it's broken due to snippets that I removed.

[raboof]

Eventually I think it'd be neat if PlayScalaClientCodeGenerator moved to play-grpc as well, since you'd expect the akka-grpc API's to stabilize and want to evolve the Play codegen along with the other play tools. I would be fine with postponing that work if it makes it easier to make smaller increments, though.

[raboof]

On this side this looks good of course ;).

Curious how an example play+akka-grpc app now looks like, but from https://github.com/playframework/play-grpc/blob/master/docs/src/main/paradox/play-framework.md that looks pretty straightforward. Nice!

[raboof]

I somehow feel this should be more prominent, but no specific suggestions, so OK for now.

[dwijnand]

Next steps from here is updating the quickstarts.