Unify work item RolledupDates and StartAndDueDate widgets

Problem

RolledupDates work item widget was created as a EE version of the StartAndDueDate widget with the roll up capability. During its creation though we didn't foresee some of the inconsistencies that keep two different widgets for the same data set could bring. Now, that we have more users testing the WorkItem epics, we're starting to see that it might be easier to manage a single widget with the EE capability as an extension, instead of two different widgets.

Proposed solution

The solution will be an iteration of multiple steps organized as tasks in this issue. The overall flow of the solution is divided in 3 main steps:

1. Add a EE extension for the StartAndDueDate - done (two tasks in this issue)
2. Update frontend to use the new StartAndDueDate extensions instead of rolled up widget - #493719 (closed)
3. Remove deprecated code - #493720 (closed)