Exempt case studies list from announcements

Announcements don't show for individual case studies. Make the list
page also skip any current announcement.

Author: Tim Bannister

layouts/case-studies/list.html

@@ -76,3 +76,6 @@ <h3>{{ .Title }}</h3>
 	<a href="{{ .page.RelPermalink }}"{{ if $isForeignLanguage }} target="_blank"{{ end }}>{{ T "main_read_about"}} {{ .page.LinkTitle }}</a>
 </div>
 {{ end }}
+{{ define "announcement" }}
+<!-- skip announcement -->
+{{ end }}