Allow for retention of line breaks in copy-to-clipboard

By changing the temporary element for holding text from a `span` to a
`pre`, line breaks in formatted strings may be retained when copying
to the clipboard

NB: This change cannot be directly tested

src/sidebar/util/copy-to-clipboard.js

@@ -11,7 +11,7 @@
  *   to copy text.
  */
 function copyText(text) {
-  const temp = document.createElement('span');
+  const temp = document.createElement('pre');
   temp.className = 'copy-text';
   temp.textContent = text;
   document.body.appendChild(temp);