Use local KathJax instead of CDN Mathjax

Summary:
This makes things like

    $\qquad\blue{y+1} \\$
    $\qquad\pink{\dfrac54 - \dfrac67y}$

display correctly

Test Plan:
load [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%2A%2AAdd%20the%20two%20expressions%20and%20combine%20like%20terms.%2A%2A%20Enter%20any%20fractions%20greater%20than%201%20as%20improper%20fractions.%5Cn%5Cn%24%5C%5Cqquad%5C%5Cblue%7By%2B1%7D%20%5C%5C%5C%5C%24%20%20%5Cn%24%5C%5Cqquad%5C%5Cpink%7B%5C%5Cdfrac54%20-%20%5C%5Cdfrac67y%7D%24%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22expression%22%2C%22options%22%3A%7B%22placeholder%22%3A%22This%20answer%20area%20is%20being%20deprecated.%20Please%20use%20the%20widgets%20in%20the%20question%20area%20for%20your%20answer.%22%2C%22value%22%3A%221%2F7y%2B9%2F4%22%2C%22form%22%3Afalse%2C%22simplify%22%3Atrue%2C%22times%22%3Afalse%2C%22functions%22%3A%5B%22f%22%2C%22g%22%2C%22h%22%5D%2C%22buttonSets%22%3A%5B%22basic%22%5D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22To%20start%20out%2C%20let%27s%20write%20these%20two%20expressions%20together%20with%20an%20addition%20sign%20between%20them.%5Cn%5Cn%24%5C%5Cqquad%5C%5Cblue%7By%2B1%7D%2B%5C%5Cpink%7B%5C%5Cdfrac54%20-%20%5C%5Cdfrac67y%7D%24%5Cn%5CnTechnically%2C%20this%20is%20all%20we%20have%20to%20do%20to%20add%20the%20expressions%2C%20but%20we%20should%20simplify%20this%20new%20expression%20by%20combining%20like%20terms.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Let%27s%20rewrite%20the%20%24-%5C%5Cfrac67y%24%20as%20%24%2B%28-%5C%5Cfrac67y%29%24%20so%20we%20can%20use%20the%20commutative%20property%20of%20addition%20to%20rearrange%20the%20terms.%5Cn%5Cn%24%5C%5Cqquad%5C%5Cblue%7By%2B1%7D%2B%5C%5Cpink%7B%5C%5Cdfrac54%20%2B%28-%20%5C%5Cdfrac67y%29%7D%24%5Cn%5Cn%28%5C%22Subtract%20%24%5C%5Cfrac67y%24%5C%22%20and%20%5C%22add%20negative%20%24%5C%5Cfrac67y%24%5C%22%20are%20two%20different%20ways%20of%20saying%20the%20same%20thing.%29%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Now%20that%20everything%20is%20written%20in%20terms%20of%20addition%2C%20let%27s%20rearrange%20the%20terms%20so%20the%20%24y%24%20terms%20are%20together%20and%20the%20constant%20terms%20are%20together.%5Cn%5Cn%24%5C%5Cqquad%5C%5Cblue%7By%7D%2B%5C%5Cpink%7B%28-%20%5C%5Cdfrac67%20y%29%7D%2B%5C%5Cblue%7B1%7D%2B%5C%5Cpink%7B%5C%5Cdfrac54%7D%20%24%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Next%2C%20let%27s%20combine%20the%20constant%20terms.%20%28We%20need%20to%20get%20a%20common%20denominator.%29%5Cn%5Cn%24%5C%5Cbegin%7Balign%7D%20%5C%5Cqquad%201%20%2B%20%5C%5Cdfrac54%20%26%3D%20%5C%5Cdfrac44%20%2B%5C%5Cdfrac54%20%5C%5C%5C%5C%5Cn%5C%5C%5C%5C%5Cn%26%3D%20%5C%5Cdfrac94%20%5C%5Cend%7Balign%7D%24%5Cn%5Cnso%20we%20can%20rewrite%20our%20expression%20as%5Cn%5Cn%24%5C%5Cqquad%5C%5Cblue%7By%7D%2B%5C%5Cpink%7B%28-%20%5C%5Cdfrac67%20y%29%7D%2B%5C%5Cdfrac94%24%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Now%20let%27s%20use%20the%20distributive%20property%20to%20combine%20the%20two%20%24y%24%20terms%20into%20a%20single%20term%20%28ignoring%20the%20%24%5C%5Cdfrac94%24%20for%20now%29.%5Cn%5Cn%24%5C%5Cbegin%7Balign%7D%20%5C%5Cqquad%20%5C%5Cblue%7By%7D%2B%5C%5Cpink%7B%28-%20%5C%5Cdfrac67%20y%29%7D%20%26%3D%20%5C%5Cblue%7B1y%7D%20-%20%5C%5Cpink%7B%20%5C%5Cdfrac67y%7D%20%5C%5C%5C%5C%5Cn%5C%5C%5C%5C%5Cn%26%3D%20%28%5C%5Cblue%7B1%7D%20-%20%5C%5Cpink%7B%20%5C%5Cdfrac67%7D%29y%20%5C%5C%5C%5C%20%5Cn%5C%5C%5C%5C%5Cn%26%3D%20%28%5C%5Cblue%7B%5C%5Cdfrac77%7D%20-%20%5C%5Cpink%7B%20%5C%5Cdfrac67%7D%29y%20%5C%5C%5C%5C%20%5Cn%5C%5C%5C%5C%5Cn%26%3D%20%5C%5Cdfrac17y%20%20%5C%5Cend%7Balign%7D%24%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22The%20final%20result%20of%20adding%20the%20%24%5C%5Cblue%7B%5C%5Ctext%7Bblue%7D%7D%24%20and%20%24%5C%5Cpink%7B%5C%5Ctext%7Bpink%7D%7D%24%20expressions%20is%5Cn%5Cn%24%5C%5Cqquad%20%5C%5Cdfrac17y%20%2B%20%5C%5Cdfrac94%24%5Cn%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%5D%7D)
See both expressions in color

Auditors: charlie