Added option to use datepicker without a separate button / icon

There is a new `datepicker_use_button` option in `BasePickerType` which defaults to true (old behaviour). If it is set to false then the extra HTML for the activation button is not rendered in `datepicker.html.twig`.

Note that the new option does not have the `dp_` prefix since it is not an option passed through to datepicker's javascript constructor.