Stop (inadvertantly) using the MathJax CDN , but re-add the accessibility code this removes

# MathJax 2.7 loads the accessibility code from its own (soon-to-be deprecated CDN); use our own CDN instead.

Author: jsharkey13

app/index.biology.html

@@ -37,7 +37,7 @@
 
     <script src="/js/lib/nprogress.js"></script>
 
-    <script type="text/javascript" src="https://cdn.isaacphysics.org/vendor/mathjax/2.7/MathJax.js?delayStartupUntil=configured"> </script>
+    <script type="text/javascript" src="https://cdn.isaacphysics.org/vendor/mathjax/2.7/MathJax.js?delayStartupUntil=configured&noContrib"> </script>
 
     <link rel="stylesheet" href="/css/app.css" />
 

app/index.chemistry.html

@@ -37,7 +37,7 @@
 
     <script src="/js/lib/nprogress.js"></script>
 
-    <script type="text/javascript" src="https://cdn.isaacphysics.org/vendor/mathjax/2.7/MathJax.js?delayStartupUntil=configured"> </script>
+    <script type="text/javascript" src="https://cdn.isaacphysics.org/vendor/mathjax/2.7/MathJax.js?delayStartupUntil=configured&noContrib"> </script>
 
     <link rel="stylesheet" href="/css/app.css" />
 

app/index.physics.html

@@ -37,7 +37,7 @@
 
     <script src="/js/lib/nprogress.js"></script>
 
-    <script type="text/javascript" src="https://cdn.isaacphysics.org/vendor/mathjax/2.7/MathJax.js?delayStartupUntil=configured"> </script>
+    <script type="text/javascript" src="https://cdn.isaacphysics.org/vendor/mathjax/2.7/MathJax.js?delayStartupUntil=configured&noContrib"> </script>
 
     <link rel="stylesheet" href="/css/app.css" />
 

app/js/app/MathJaxConfig.js

@@ -24,9 +24,10 @@ define([], function() {
 		//AMS.labels = {};
 	}
 
-	// Allow inline maths with single $s
-
+	// Specify where to find the official Accessibility extension:
+	MathJax.Ajax.config.path["a11y"] = "https://cdn.isaacphysics.org/vendor/mathjax/mathjax-a11y-v1.1.0";
 
+	// Allow inline maths with single $s, define Isaac macros:
 	MathJax.Hub.Config({
 
 		config: ["TeX-AMS_HTML.js"],
@@ -54,7 +55,7 @@ define([], function() {
 				"units": ["\\rm{#1}",1],
 				"standardstate": ["\\mathbin{\u29B5}",0],
 			},
-			extensions: ["mhchem.js"],
+			extensions: ["mhchem.js", "[a11y]/accessibility-menu.js"],
 		},
 	  "HTML-CSS": {
 	    availableFonts: [],