Add MathZoom.js to MathJax for real

Summary:
MathJax was trying to load MathZoom.js even though we removed it from
the config file and rebuilt KathJax.  In this diff we include so that
MathJax is happy when it tries to load it.

Test Plan:
- load http://localhost:8080/math/cc-eighth-grade-math/cc-8th-systems-topic/cc-8th-systems-overview/e/systems_of_equations_with_substitution?mp-r-id=GjFgG84=
- verify that there's no errors in the console and there's no red warning bar at the top of the exercise

Reviewers: emily

Reviewed By: emily

Differential Revision: https://phabricator.khanacademy.org/D21613

build/kathjax-config.js

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

build/kathjax.py

@@ -9,6 +9,7 @@
 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-f44d133888282e24c89e102b0a3fa99c", waitForMathJaxReady);
+        loadScript(urlBase + "third_party/MathJax/2.1/MathJax.js?config=KAthJax-186c6982df2f62edb8559effd15b8508", waitForMathJaxReady);
     }
 
     function waitForMathJaxReady() {

lint_blacklist.txt

@@ -4,5 +4,6 @@ local-only/**
 genfiles/**
 third_party/**
 third_party/MathJax/2.1/config/*
+third_party/MathJax/2.1/extensions/*
 build/third_party/**
 test/**