Add xdan-datetimepicker-rails and datetime_picker input

No tests yet. This is pretty much what I've implemented in another project.

I haven't tried using this in place of what I have in my project yet.

[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.