Use due date widget in the refactored boards sidebar.

In board_content_sidebar.vue, we should replace board_sidebar_due_date.vue with sidebar_date_widget.vue.

sidebar_date_widget.vue is the recently created due date widget - board_sidebar_due_date.vue should be deleted after this task is done.

Todos

• Replace board_sidebar_due_date.vue with sidebar_date_widget.vue in board_content_sidebar.vue.
• ⚠ Make sure to update feature specs for due dates (if the coverage is missing).