Update to higlights and migration guide for 2.3

* Revised Activator section
* Expanded info on Java performance
* Split WS changes into separate library and security changes
* Java examples
* Mentioned SSLEngine change
* Migration for sbt version
* Mention needed change to Play plugin version
* Mention need for explicit scalaVersion setting
Merge pull request #2791 from richdougherty/release-docs

Update to higlights and migration guide for 2.3