add meta robots tag to deploy previews to make extra sure they aren't indexed

layouts/partials/head/meta.html

@@ -1,5 +1,6 @@
 <meta charset="utf-8">
 {{ hugo.Generator }}
+{{ if ne hugo.Environment "production" }}<meta name="robots" content="noindex, nofollow">{{ end }}
 <title>{{ .Scratch.Get "plainTitle" }}{{ if not .IsHome }} – {{ .Site.Title }}{{ end }}</title>
 <meta name="description" content="{{ with .Description }}{{ . }}{{ else }}{{ .Site.Params.description }}{{ end }}">
 {{ with .Site.Author.name }}<meta name="author" content="{{ . }}">{{ end }}