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

uhm.. only references to `formatHour`:

```
$ ag 'formatHour' apps/calendar/
apps/calendar/js/calc.js
53:    formatHour: function(hour) {

apps/calendar/test/unit/calc_test.js
71:  suite('#formatHour', function() {
92:      var result = subject.formatHour(7);
97:      var result = subject.formatHour(23);

apps/calendar/test/unit/views/day_based_test.js
483:        displayHour: Calendar.Calc.formatHour(hour),
677:        Calendar.Calc.formatHour(hour),
```