message: Remove fickle index-based traversing
  + change fake link to proper <button>
diff: @@ -7,20 +7,17 @@ window.addGistCopyButton = () => {
  if ($('.copy-btn').length > 0) {
  return;
  }
  - const $gistsSibling = $('.file-actions > .btn.btn-sm');
  - for (let i = 0; i < $gistsSibling.length; i++) {
  - const commonContainer = $('.file')[i];
  - const isSourceCodeClass = commonContainer.children[1].classList.contains('blob-wrapper');
  - const gistUri = $gistsSibling[i].href;
  - if (isSourceCodeClass) {
  - $(`<a href="${gistUri}" class="btn btn-sm copy-btn gist-copy-btn">Copy</a>`).insertBefore($gistsSibling[i]);
  - }
  - }
  + $('.blob-wrapper').each((i, blob) => {
  + const actionsParent = blob.parentNode.querySelector('.file-actions');
  + const $btn = $(`<button class="btn btn-sm copy-btn gist-copy-btn">Copy</button>`);
  + $btn.data('blob', blob);
  + $btn.prependTo(actionsParent);
  + });
  $(document).on('click', '.copy-btn', e => {
  e.preventDefault();
  - const fileContents = $(`#${e.currentTarget.offsetParent.id}`).find('.js-file-line-container')[0].innerText;
  + const fileContents = $(e.currentTarget).data('blob').innerText;
  utils.copyToClipboard(fileContents);
  });
  };
  