Add the change state value to ChangeEvent docs

[morucci]

This will enable to filter results of metrics based on change events according to the change status. For instance, currently complex metrics like "Changes lifecycle stats" and "Changes review stats" displays empty result when we set a query filter based on the chage state.

• [ ] Add the changeState to all ChangeEvent docs
• [ ] Add a database migration process to add the new field to existing documents