Code highlighting with markdown in comments is broken

Issue Summary

When editing a comment with markdown code, the highlighting is working, but once the comment is saved, all highlighting is gone. See attached picture for reference.

Steps to Reproduce

1. Step 1: Add a comment in the pull request like the below:

   ``` C++
   auto powerEnable = isi->GetTisPowerEnable().lock();
   if (powerEnable) {
       *powerEnable = false;
       *powerEnable = true;
   }
   ```
   

   You see some of the word highlighted before you post the comment.

2. Step 2
   When you click on the Comment button, you see that the syntax highlighting is gone.

   Expected Results

   The syntax highlighting should remain after the comment is posted

   Actual Results

   The syntax highlighting is gone after the comment is posted even though it shows up while drafting the comment

Workaround

Use a different alias for the code highlighting mode as defined in CodeMirror's codebase (look if the highlighting mode you need has an alias, if it does, use one of the listed aliases). For instance in the noted case above, a working highlighting mode is cpp.