API Default to "yyyy-MM-dd" for date format

The motivation for this was to avoid using "MMM" in particular, since it causes inconsistencies in month
names between jQuery UI and Zend_Locale_Format.
One example of that was "mar." (jQuery UI( vs. "mars" (Zend_Locale) as a danish short month name.

This means if you set a default locale of `de_DE` for example,
the date format will still be `yyyy-MM-dd` (rather than `dd.MM.yyyy`).
You still get that one as a suggestion in the member profile,
and can set it as a global default through `i18n.date_format` config API,
its just not auto detected any longer. This also makes `i18n::get_date_format()`
obsolete, since we can now simply use the Config API without any dynamic calls.

I've implemented the same changes for consistency in `time_format` as well.

Fixes https://github.com/silverstripe/silverstripe-cms/issues/544