Ensure legacy Epic views/API return the same dates from the WorkItem view/API

The Problem

Roadmap relies on Epic API and we plan to move it to use Work Item API as a part of #478011, but until we do that, the expectation is that there won't be a significant mismatch of dates between both the APIs especially because other attributes of Epics are consistent between old and new experience.

Proposed Solution

Overwrite Epic dates methods to use the same logic described in &11409 (closed). Since these rules are already coded on WorkItems::Widget::StartAndDueDate, we can use that logic within the epic itself, something like:

class Epic
  # ...

  def start_date
    start_and_due_date_widget.start_date
  end
  alias_method :start_date_fixed, :start_date

  def start_date_is_fixed
    start_and_due_date_widget.fixed?
  end
  alias_method :start_date_is_fixed?, :start_date_is_fixed

  def due_date
    start_and_due_date_widget.due_date
  end
  alias_method :end_date, :due_date
  alias_method :due_date_fixed, :due_date

  def due_date_is_fixed
    start_and_due_date_widget.fixed?
  end
  alias_method :due_date_is_fixed?, :due_date_is_fixed

  private

  def start_and_due_date_widget
    @start_and_due_date_widget ||= work_item.get_widget(:start_and_due_date)
  end
  # ...
end