Add latex support

Adds latex highlighting support via https://www.npmjs.com/package/katex
as requested in
https://mattermost.uservoice.com/forums/306457-general/suggestions/10160757-add-latex-support

web/templates/head.html

@@ -24,6 +24,7 @@
     <link rel="stylesheet" href="/static/css/bootstrap-colorpicker.min.css">
     <link rel="stylesheet" href="/static/css/styles.css">
     <link rel="stylesheet" href="/static/css/google-fonts.css">
+    <link rel="stylesheet" href="/static/css/katex.min.css">
     <link rel="stylesheet" class="code_theme" href="">
 
     <link id="favicon" rel="icon" href="/static/images/favicon.ico" type="image/x-icon">
@@ -38,6 +39,7 @@
     <script src="/static/js/perfect-scrollbar-0.6.7.jquery.min.js"></script>
     <script src="/static/js/jquery-dragster/jquery.dragster.js"></script>
     <script src="/static/js/babel-es6-polyfill.min.js"></script>
+    <script src="/static/js/katex.min.js"></script>
 
     <style id="antiClickjack">body{display:none !important;}</style>