ensure that mathjax scripts render only when MATHJAX env value is true

lib/views/layout/layout.html

@@ -35,7 +35,7 @@
 
   <!-- jQuery, emojione -->
   <script src="https://cdn.jsdelivr.net/combine/npm/emojione@3.1.2,npm/jquery@3.3.1"></script>
-  {% if true %}
+  {% if local_config.env.MATHJAX %}
     <!-- Mathjax -->
     <script type="text/x-mathjax-config" async>
       MathJax.Hub.Config({