Bug 1054178 - [Gaia][calendar] support 24 hour time format

can we use template strings going forward?

```js
if (this.arg('isAllDay')) {
  return '<div class="all-day" data-l10n-id="hour-allday"></div>';
}
var startTime = formatTime(this.arg('startTime'));
var endTime = formatTime(this.arg('endTime'));
return `<div class="start-time">${startTime}</div>
  <div class="end-time">${endTime}</div>`;
```