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

all the formats follows the same structure `id[12|24]`, so we could write it like this instead:

```js
var elements = document.querySelectorAll('[data-l10n-date-format*="12"]');
// ...
element.dataset.l10nDateFormat = element.dataset.l10nDateFormat.replace('12', '24');
```

the main advantage is that we could keep adding more formats without touching these lines..