API Removed auto-detection for i18n date/time formats

Default to "yyyy-MM-dd" for date format, and "H:mm" for time_format.
Switched to config API for setting/getting values.

Avoid using "MMM" in particular, since it causes
inconsistencies in month names between jQuery UI and Zend_Locale_Format.

Fixes https://github.com/silverstripe/silverstripe-cms/issues/544