refactor: move creation of 'Copy' button from client to server

* https://github.com/cheeriojs/cheerio#append-content-content--
  * cheerio doesn't have .createElement() function
    - https://code.tutsplus.com/tutorials/jquery-14-released-the-15-new-features-you-must-know--net-8590

themes/typing/scripts/copy-button.js

@@ -0,0 +1,14 @@
+// Add "Copy" button to code snippet
+
+const cheerio = require('cheerio')
+
+hexo.extend.filter.register('after_render:html', (str) => {
+  const $ = cheerio.load(str)
+
+  // Avoid duplicate button
+  if ($('button').parent().hasClass('code')) return
+
+  $('.code').append('<button class="copy-button">Copy</button>')
+
+  return $.html()
+})

themes/typing/source/js/typing.js

@@ -6,17 +6,6 @@ searchClick.onclick = function() {
   '&group_id=&project_id=8306723&search_code=true&repository_ref=master')
 }
 
-// Add "Copy" button to code snippet
-const code = document.getElementsByClassName('code')
-
-Array.from(code).forEach((element) => {
-  const button = document.createElement('button')
-  button.className = 'copy-button'
-  button.textContent = 'Copy'
-
-  element.appendChild(button)
-})
-
 /*
 * Copy button and Cloudinary functions.
 * Following functions only execute after