Add iterations and resource events to Import/Export & GitLab Migration

Release notes

GitLab Migration migrates now Iteration cadences of groups. If you've been using Iteration cadences to automate creating your iterations or automatically roll over incomplete issues to the next iteration, this information and configuration will be migrated along with your groups.

Documentation: https://docs.gitlab.com/ee/user/group/import

Original problem

We currently do not include iterations in our import / export. This also applies to resource_iteration_events, resource_milestone_events, and resource_state_events and iteration board lists.

We need the resource events so that burndown / burnup charts are imported / exported correctly.

I'm not sure how this would work with project exports though because iterations are currently at the group level. Does this mean we cannot include issue iterations and project board iteration lists in our project exports?

Proposed solution

Iterations are already migrated as part of the GitLab Migration. And so are Issues. What we need to add is the connection. We already do something similar for Epics and Issues (for example).