Updated the way Play uses application.secret

Fixes #1775.

* Play now generates an application secret that is stable to the place
  where the application is running from in dev and test mode.
* Play will fail to load if application secret is not set or changeme in
  production.
* Created new SBT tasks for generating and updating the application
  secret.
* Wrote new docs on the application secret, how configure it, and best
  practices.
* Updated production documentation to refer to activator.
Merge pull request #1775 from jroper/application-secret

sbt task for generating a new application.secret