Project Import does not map issues to existing sprints

Summary

Sprint and Ranking data can be imported via Project Import, as per JRA-28748 however it does not import issues to any existing sprints in the target instance. Instead, new sprints are created per project import so there will be duplicated sprints with the same sprint name.

Steps to Reproduce

1. Configure the source instance to have 3 projects with Scrum boards
2. In Board 1, edit the filter to show issues from the other 2 boards
   
3. Create a sprint with issues from 3 projects and start the sprint
   
4. Generate an XML backup and configure target instance as per step 1
5. Run Project Import in target instance for all 3 projects

Expected Results

Issues should be mapped to the same sprint after project import

Actual Results

Issues are mapped to 3 different sprints with the same sprint name

Workaround

1. Manually map imported issues into one sprint and remove the other sprints
2. Re-configure board filter in Board 1

The workaround might affect the data in Reports, specifically when the sprint has been started in the source instance