Use resource events to track iteration changes

To make it easier to generate the burnup and burndown charts, we need to track iteration changes as resource events in the database.

This is similar to what we do with milestones.

To Do

• create db table (!37617 (merged))
• Add the iteration events model (!37617 (merged))
• track the iteration change events (!37620 (merged))
• enable track_iteration_change_events FF by default (!40841 (merged))

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