GitHub Importer - Migrate issue 'closed/reopened' events

Release Notes

We continue to improve the GitHub project importer and add more meta data to it. In this milestone we added events history of closing and reopening issues, so related events in GitHub issues will be migrated to GitLab issues.

Problem to solve

When migrating projects from GitHub to GitLab, the issues are imported including any comments. However, any history/events are not migrated over. This means that, after the migration, the users would not be able to see who added/removed labels, milestones, assignees, etc.

For some users this is a dealbreaker that would prevent them from migrating to GitLab.

Proposed solution

During project import from GitHub, for each imported issue, check if any events of type closed or reopened exist for that issue and create corresponding events in GitLab.