Add xdan-datetimepicker-rails and datetime_picker input

Adds [XDSoft's DateTime picker](http://xdsoft.net/jqplugins/datetimepicker/) as the `combined_date_time_picker` input for forms.

[IRC](https://botbot.me/freenode/activeadmin/msg/12028495/) prompting this PR.

Please try this out and see if there's any tweaks or suggestions to better implement this, or if you notice any bugs.

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There is a `DatepickerInput`, but I can't find a `DateTimePickerInput`