Bug 973441 - [settings] refactor Date & Time panel with AMD pattern
- use class instead of id
- fix tz_select loading
- use SettingsCache
- re-organize keys
- implement attachListeners/detachListeners
- add unit test
- have a DateTime module for setting date, time, and time zone
- apply observable to whole modules/date_time
- move datePickerChange/timePickerChange into module
- move event handler to panel and autoUpdateDate/Time to DateTime module
- move UI to panel
- auto date/time observer
- fix auto switch
- add formatDate and formatTime