Change StartAndDueDate widget to use WorkItems::DatesSource as its database model to write and read

Problem

With the introduction of the RolledupDates we also introduced WorkItems::DatesSource model to be the work item single source of truth for dates. However, the StartAndDueDate widget still writing dates directly on the issues table.

To make matters worse, there's also some confusion between the RolledupDates and the StartAndDueDate widgets. Now with more knowledge, we see that the RolledupDates widget should be actually just an EE extension for the already existing StartAndDueDate.

Proposed solution

To simplify all that, we need to:

  1. Ensure a single source of truth for the work items dates fields in the database: #469379 (closed)
  2. Ensure that StartAndDueDate widget is using the WorkItems::DatesSource as its database model (this issue)
  3. Change the RolledupDates widget to be an EE extension of the StartAndDueDate widget

Related work:

Edited by Kassio Borges