Disable copy-code button for mermaid diagrams

It removes the copy-code button for mermaid. The
button was added only for markdown code blocks.

Changelog: fixed

app/assets/javascripts/behaviors/copy_code.js

@@ -30,6 +30,7 @@ class CopyCodeButton extends HTMLElement {
 
 function addCodeButton() {
   [...document.querySelectorAll('pre.code.js-syntax-highlight')]
+    .filter((el) => el.attr('lang') !== 'mermaid')
     .filter((el) => !el.closest('.js-markdown-code'))
     .forEach((el) => {
       const copyCodeEl = document.createElement('copy-code');