Unscope preview* settings that are used in both previewSite and previewAuto

[2m]

Currently previewFixedPort and previewLaunchBrowser are scoped under previewSite but are used in both previewSite and previewAuto tasks:

https://github.com/sbt/sbt-site/blob/develop/src/main/scala/com/typesafe/sbt/site/SitePreviewPlugin.scala#L14-L15

These settings should be unscoped in the autoImport.