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/react/.eslintrc

@@ -6,7 +6,7 @@
             "modules": true,
             "classes": true,
             "arrowFunctions": true,
-            "defaultParams": true, 
+            "defaultParams": true
         },
         "parser": "babel-eslint",
         "plugins": [
@@ -22,7 +22,8 @@
             "React": false,
             "ReactDOM": false,
             "ReactBootstrap": false,
-            "Chart": false
+            "Chart": false,
+            "katex": false
         },
         "rules": {
             "comma-dangle": [2, "never"],

web/react/utils/markdown.jsx

@@ -69,6 +69,15 @@ class MattermostMarkdownRenderer extends marked.Renderer {
             usedLanguage = 'xml';
         }
 
+        if (usedLanguage && (usedLanguage === 'tex' || usedLanguage === 'tex')) {
+            try {
+                var html = katex.renderToString(TextFormatting.sanitizeHtml(code), {throwOnError: false, displayMode: true});
+                return '<div class="post-body--code tex">' + html + '</div>';
+            } catch (e) {
+                return '<div class="post-body--code">' + TextFormatting.sanitizeHtml(code) + '</div>';
+            }
+        }
+
         if (!usedLanguage || highlightJs.listLanguages().indexOf(usedLanguage) < 0) {
             let parsed = super.code(code, usedLanguage);
             return '<div class="post-body--code"><code class="hljs">' + TextFormatting.sanitizeHtml($(parsed).text()) + '</code></div>';