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

would be good if we could call something like:

```js
// returns the .display-hour <span> as a string
dateSpan({
  date: date,
  format: 'week-hour-format',
  className: 'display-hour'
});
```

and not have to deal with the conditionals and all the `navigator.mozL10n.get` and `Calendar.App.dateFormat.localeFormat` calls.. let's keep things DRY.