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

Looks like we could not list `Calendar.App.dateFormat.localeFormat` at the top. Because we load `app.js` after `date_span.js` is loaded in the `index.html` file[1][2].

[1] https://github.com/mozilla-b2g/gaia/blob/master/apps/calendar/index.html#L112
[2] https://github.com/mozilla-b2g/gaia/blob/master/apps/calendar/index.html#L71-L83