Use a modal dialog box for documentation warnings

[wsargent]

There are several warning notices that kick into action on the site:

https://www.playframework.com/documentation/2.5.x/Home: "You are viewing the documentation for the 2.5.x release series. The latest stable release series is 2.6.x."

https://www.playframework.com/documentation/1.4.x/home: "You are viewing the documentation for Play 1. The documentation for Play 2 is here."

These warning are above the search bar, and have visual styling that is hard to distinguish from the rest of the page. It would be better to turn this into a modal dialog that covers the rest of the page and must be explicitly clicked through and ignored (potentially with cookie for persistent setting) so that it can't be skipped past.

[marcospereira]

I think we can go the Akka way here:

1. For non-supported versions: https://doc.akka.io/docs/akka/2.4.10/scala.html
2. For outdated versions: https://doc.akka.io/docs/akka/2.5.1/scala.html

What do you think?

[eed3si9n]

FYI @xuwei-k reported on Twitter (https://twitter.com/xuwei_k/status/1041313148035203073) about a new user in Japan following Play 2.4 guide and doing:

$ activator new my-first-app play-scala

Having the banner would've helped I think.

[eed3si9n]

Ref https://github.com/playframework/playframework/issues/8609