WebJar CDN path issues

As initially reported here: https://github.com/sbt/sbt-web/issues/39

We now avoid the webjar locator for CDN mapping as the version of the WebJar that it returned wasn't the right one for our needs. As a consequence the WebJars were being mapped incorrectly for the CDN.

CDN configuration is also enhanced so it also receives a mapping of WebJar organisations to CDNs. Thus you can now have many CDNs declared.