don't syntax highlight languages that begin with "_"

src/HtmlUtils.tsx

@@ -184,7 +184,7 @@ const transformTags: sanitizeHtml.IOptions["transformTags"] = { // custom to mat
         if (typeof attribs.class !== 'undefined') {
             // Filter out all classes other than ones starting with language- for syntax highlighting.
             const classes = attribs.class.split(/\s/).filter(function(cl) {
-                return cl.startsWith('language-');
+                return cl.startsWith('language-') && !cl.startsWith('language-_');
             });
             attribs.class = classes.join(' ');
         }

src/components/views/messages/TextualBody.js

@@ -107,7 +107,7 @@ export default createReactClass({
                         } else {
                             // Only syntax highlight if there's a class starting with language-
                             const classes = blocks[i].className.split(/\s+/).filter(function(cl) {
-                                return cl.startsWith('language-');
+                                return cl.startsWith('language-') && !cl.startsWith('language-_');
                             });
 
                             if (classes.length != 0) {

src/editor/deserialize.ts

@@ -64,7 +64,7 @@ function parseCodeBlock(n: HTMLElement, partCreator: PartCreator) {
     let language = "";
     if (n.firstChild && n.firstChild.nodeName === "CODE") {
         for (const className of (<HTMLElement>n.firstChild).classList) {
-            if (className.startsWith("language-")) {
+            if (className.startsWith("language-") && !className.startsWith("language-_")) {
                 language = className.substr("language-".length);
                 break;
             }