Reuse date widget in epic sidebar

As part of epic boards sidebar (#231401 (closed)), a date widget was created, supporting Inherited dates for epics. This component should replace date selectors in epic sidebar.

Widget is app/assets/javascripts/sidebar/components/date/sidebar_date_widget.vue and an example of its use exists in ee/app/assets/javascripts/boards/components/epic_board_content_sidebar.vue.