Merge branch 'katex-math-license' into 'master'

Added KaTeX license and procedure to build it for Gitlab

## What does this MR do?
Adds KaTeX license and the procedure for building KaTeX for gitlab. 

## Are there points in the code the reviewer needs to double check?
Nope.

## Why was this MR needed?
Requested by @stanhu

## Screenshots (if relevant)
Not relevant. 

## Does this MR meet the acceptance criteria?

- [X] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [X] API support added
- Tests
  - [X] Added for this feature/bug
  - [X] All builds are passing
- [X] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [X] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [X] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [X] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !8132

vendor/assets/javascripts/katex.js

+/*
+ The MIT License (MIT)
+
+ Copyright (c) 2015 Khan Academy
+
+ This software also uses portions of the underscore.js project, which is
+ MIT licensed with the following copyright:
+
+ Copyright (c) 2009-2015 Jeremy Ashkenas, DocumentCloud and Investigative
+ Reporters & Editors
+
+ Permission is hereby granted, free of charge, to any person obtaining a copy
+ of this software and associated documentation files (the "Software"), to deal
+ in the Software without restriction, including without limitation the rights
+ to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ copies of the Software, and to permit persons to whom the Software is
+ furnished to do so, subject to the following conditions:
+
+ The above copyright notice and this permission notice shall be included in all
+ copies or substantial portions of the Software.
+
+ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ SOFTWARE.
+ */
+
+/*
+ Here is how to build a version of KaTeX that works with gitlab.
+
+ The problem is that the standard procedure for changing font location doesn't work for the empty string.
+
+ 1. Clone KaTeX. Anything later than 4fb9445a9 (is merged into master) will do.
+ 2. make (requires node)
+ 3. sed -i 's,fonts/,,' build/katex.css
+ 4. Copy build/katex.js, build/katex.css and fonts/* to gitlab.
+*/
+
 (function(f){if(typeof exports==="object"&&typeof module!=="undefined"){module.exports=f()}else if(typeof define==="function"&&define.amd){define([],f)}else{var g;if(typeof window!=="undefined"){g=window}else if(typeof global!=="undefined"){g=global}else if(typeof self!=="undefined"){g=self}else{g=this}g.katex = f()}})(function(){var define,module,exports;return (function e(t,n,r){function s(o,u){if(!n[o]){if(!t[o]){var a=typeof require=="function"&&require;if(!u&&a)return a(o,!0);if(i)return i(o,!0);var f=new Error("Cannot find module '"+o+"'");throw f.code="MODULE_NOT_FOUND",f}var l=n[o]={exports:{}};t[o][0].call(l.exports,function(e){var n=t[o][1][e];return s(n?n:e)},l,l.exports,e,t,n,r)}return n[o].exports}var i=typeof require=="function"&&require;for(var o=0;o<r.length;o++)s(r[o]);return s})({1:[function(require,module,exports){
 /* eslint no-console:0 */
 /**

vendor/assets/stylesheets/katex.css

+/*
+The MIT License (MIT)
+
+Copyright (c) 2015 Khan Academy
+
+This software also uses portions of the underscore.js project, which is
+MIT licensed with the following copyright:
+
+Copyright (c) 2009-2015 Jeremy Ashkenas, DocumentCloud and Investigative
+Reporters & Editors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+*/
+
+/*
+ Here is how to build a version of KaTeX that works with gitlab.
+
+ The problem is that the standard procedure for changing font location doesn't work for the empty string.
+
+ 1. Clone KaTeX. Anything later than 4fb9445a9 (is merged into master) will do.
+ 2. make (requires node)
+ 3. sed -i 's,fonts/,,' build/katex.css
+ 4. Copy build/katex.js, build/katex.css and fonts/* to gitlab.
+*/
+
 @font-face {
   font-family: 'KaTeX_AMS';
   src: url('KaTeX_AMS-Regular.eot');