Resolve ""Retry" button opens about:blank in new tab"

Closes #829 (closed)

aleksis/core/templates/500.html

@@ -16,7 +16,7 @@
           {% endblocktrans %}
         </p>
         {% include "core/partials/admins_list.html" %}
-        <a href="javascript:window.location.reload()" class="btn green waves-effect waves-light">
+        <a onClick="window.location.reload();" class="btn secondary waves-effect waves-light">
           <i class="material-icons left">refresh</i>
           {% trans "Retry" %}
         </a>