Add a Completed Date column to the To-Do List view

Glen Ditchfieldrequested to merge
completed-date into master

Add a read-only "Completed Date" column to the To-Do List view. It is potentially useful when billing for activities. It is draggable and hideable like the other columns. Like the Start and Due Date columns, it only displays the date, but the exact time is visible in the tool tip.

CCBUG: 374774

Merge request reports