Allow date format to supported group of built-in patterns

Until now 'named dates' like `dateOptionalTime` could not be used as a group
of dates. This patch allows it to group it arbitrarily like this:

* yyyy/MM/dd HH:mm:ss||yyyy/MM/dd||dateOptionalTime
* dateOptionalTime||yyyy/MM/dd HH:mm:ss||yyyy/MM/dd
* yyyy/MM/dd HH:mm:ss||dateOptionalTime||yyyy/MM/dd
* date_time||date_time_no_millis

Closes #2132