Fix for https://github.com/keystonejs/keystone/issues/238 'Datetime: 'default: Date.Now' only works on the date field'

The issue is caused by template for date field using '.btn-set-today' and template for datetime field using '.btn-set-now'.

public/js/common/ui.js only adds click handler for btn-set-today which makes the button work on date but not on datetime fields.

I've added following bit of code to make 'Now' button work correctly on datetime fields updating both date and time portions:
```
$('.btn-set-now').click(function() {
    $(this).prevAll('.ui-datepicker:first').pikaday('setDate', new Date());
    $(this).prevAll('.time').val(moment().format('HH:mm:ss'));
});
```