Show a dismissable option to add The Session to the home screen (only shown to logged in users).

beforeInstallPrompt.js

(function (win, doc) {
  win.addEventListener('beforeinstallprompt', function (e) {
    e.preventDefault();
    var deferredPrompt = e;
    var insertionPoint = doc.querySelector('main .contents');
    insertionPoint.insertAdjacentHTML('afterbegin',`
      <div class="feedback" role="dialog" aria-labelledby="homescreen">
      <h2 id="homescreen">Install The Session</h2>
      <p>Would you like to add The Session to your home screen?</p>
      <button class="back">No, thanks</button>
      <button class="aside">Yes, please!</button>
      </div>
      <br>
    `);
    var dialog = insertionPoint.querySelector('.feedback');
    dialog.querySelector('button.back').addEventListener('click', function (e) {
      dialog.remove();
      doc.cookie = 'addtohomescreen=false;expires="Tue, 19 Jan 2038 03:14:07 GMT";path=/';
    }, false);
    dialog.querySelector('button.aside').addEventListener('click', function (e) {
      deferredPrompt.prompt();
      deferredPrompt.userChoice.then(function (choiceResult) {
        if (choiceResult.outcome === 'accepted') {
          dialog.remove();
          doc.cookie = 'addtohomescreen=true;expires="Tue, 19 Jan 2038 03:14:07 GMT";path=/';
        }
      });
    }, false);
  }, false);
}(this, this.document));