Eliminate the Activity Due Date field for case activities

Eliminate the Activity Due Date field and just have one field for case activities (as we do for regular activities).
Major changes include :

1. remove Due Date from forms, selectors, report output and queries

• need to rework the selector "select" query for case activities (in dashboard and jquery grid in manage case).
• the case level selector has Next Scheduled activity in it - and that will need to be based on next "activity_date_time" where status = Scheduled
  rather than next "due_date_time" where status = Scheduled.

1. change xmlProcessor to populate activity_date_time rather than due_date_time when auto-populating activities from a timeline (and to use
   activity_date_time for offset calculations)

1. if activity_date_time IS NULL and due_date_time IS NOT NULL, then set activity_date_time = due_date_time
   (before we drop the column)