Completing sprints moves all issues that weren't done to the next future sprint

Description

User would like to choose whether the issues (that weren't done in the Sprint that was closed) go to the next Future Sprint or go back to the Backlog.

How it is

1. Create a Board and 2 sprints
2. Fill the first Sprint with a couple of tickets and start it
3. Complete some issues (not all) and complete the sprint
4. The issues that weren't done during the Sprint will be automatically put to the next Sprint

How the user would like it to be

The user should be able to configure if he wants the following behaviour:

1. Create a Board and 2 sprints
2. Fill the first Sprint with a couple of tickets and start it
3. Complete some issues (not all) and complete the sprint
4. The issues that weren't done during the Sprint will be automatically put to the next Sprint

Or this behaviour:

1. Create a Board and 2 sprints
2. Fill the first Sprint with a couple of tickets and start it
3. Complete some issues (not all) and complete the sprint
4. The issues that weren't done during the Sprint will be automatically put back to the backlog