datetime-picker.js: Append flatpickr container not to body

The body isn't cleared without a full reload. This way the
flatpickr container resides in the same container as the
input's form.

public/js/icinga/behavior/datetime-picker.js

@@ -64,6 +64,7 @@
             var dateTimeFormatter = _this.createFormatter(! disableDate, enableTime);
             var options = {
                 locale: _this.loadFlatpickrLocale(),
+                appendTo: this.form.parentNode,
                 altInput: true,
                 enableTime: enableTime,
                 noCalendar: disableDate,