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

since we have template strings let's not do concatenation anymore!

```js
var l10n = allDay ? 'data-l10n-id="hour-allday"' : `data-l10n-date-format="${format}"`;
return `<span class="${className}" data-date="${date}" ${l10n}>
    ${displayHour}
  </span>`;
```