Add MathZoom.js extension to third_party MathJax.

Summary:
Some browsers recently started request MathZoom.js recently.  I was unable
to determine why this started so I've decided to include MathZoom.js.

Test Plan:
- update submodule in webapp and deploy
- verify that 404s for MathZoom.js in the logs disappear.

Reviewers: emily

Reviewed By: emily

Subscribers: alpert

Differential Revision: https://phabricator.khanacademy.org/D21557

build/kathjax-config.js

@@ -2,7 +2,7 @@ MathJax.Hub.Config({
     messageStyle: "none",
     skipStartupTypeset: true,
     jax: ["input/TeX","output/HTML-CSS"],
-    extensions: ["tex2jax.js","MathZoom.js"],
+    extensions: ["tex2jax.js"],
     TeX: {
         extensions: [
             "AMSmath.js",

build/kathjax.py

@@ -9,7 +9,6 @@
 pack = [
     'extensions/MathEvents.js',
     'extensions/MathMenu.js',
-    'extensions/MathZoom.js',
     'extensions/TeX/AMSmath.js',
     'extensions/TeX/AMSsymbols.js',
     'extensions/TeX/noErrors.js',

khan-exercise.js

@@ -2143,7 +2143,7 @@ function loadMathJax() {
     if (window.MathJax) {
         waitForMathJaxReady();
     } else {
-        loadScript(urlBase + "third_party/MathJax/2.1/MathJax.js?config=KAthJax-eaf1d9cafdc2fc68897cf72d30d67d5a", waitForMathJaxReady);
+        loadScript(urlBase + "third_party/MathJax/2.1/MathJax.js?config=KAthJax-f44d133888282e24c89e102b0a3fa99c", waitForMathJaxReady);
     }
 
     function waitForMathJaxReady() {

lint_blacklist.txt

@@ -3,5 +3,6 @@
 local-only/**
 genfiles/**
 third_party/**
+third_party/MathJax/2.1/config/*
 build/third_party/**
 test/**