Fix duplicate class attr in optin e-mail. Closes #524.

static/email-templates/subscriber-optin-campaign.html

@@ -12,6 +12,6 @@
     {{ end }}
 </ul>
 <p>
-    <a class="button" {{ .OptinURLAttr }} class="button">{{ L.Ts "email.optin.confirmSub" }}</a>
+    <a class="button" {{ .OptinURLAttr }}>{{ L.Ts "email.optin.confirmSub" }}</a>
 </p>
 {{ end }}