use datepicker in admins

[dbu]

the publish workflow admin extension allows to set publish start and end date. http://sandbox.cmf.symfony.com/en/admin/app/demoseocontent/cms/content/home/edit?tl=en

currently this is shown as 3 separate dropdowns, which is not elegant. afaik sonata provides a type with some nice js datepicker. we should use that instead.

[wouterj]

More information about this form type: https://sonata-project.org/bundles/core/master/doc/reference/form_types.html#sonata-type-date-picker-and-sonata-type-datetime-picker