harmonized external_url variable, removed url_external

layouts/_default/single.html

@@ -24,8 +24,6 @@
         <div class="tags-and-button">
             {{ if .Params.external_url }}
             <div class="article-external-button"><a href="{{ .Params.external_url }}" target="_blank" class="btn">Visit Website</a></div>
-            {{ else if .Params.url_external }}
-            <div class="article-external-button"><a href="{{ .Params.url_external }}" target="_blank" class="btn">Visit Website</a></div>
             {{ end }}
             {{ if .Params.tags }}
             <div class="tags">