TodoService should not call `.select(&:id)` on todos, because this is
bad performance. So instead use sub-queries, which will result in a
single SQL query to the database.

https://docs.gitlab.com/ee/development/sql.html#plucking-ids

This reverts merge request !10845

TodoService should not call `.select(&:id)` on todos, because this is
bad performance. So instead use sub-queries, which will result in a
single SQL query to the database.

https://docs.gitlab.com/ee/development/sql.html#plucking-ids

Added the ability to 'Undo mark all as done' todos marked as complete with 'Mark all as done'
in the 'Todo' tab of the Todo dashboard.
The operation undos only the todo previously marked as done with the 'Mark al as done' button.