Implementation/72849 make drag and drop work between sprints and version backlogs

[EinLama]

Ticket

https://community.openproject.org/wp/72849

Note: We have a name clash. With Sprint, I mean the legacy Sprint inheriting from Version. For the new style sprint, I will explicitly write Agile::Sprint.

Drag and drop works:

• From one Sprint to another
• From one Agile::Sprint to another
• From Agile::Sprint to Sprint
• From Sprint to Agile::Sprint

When moving a Story from any sprint type to another, we try to retain the Version assigned to the Work Package. The rule is: if the assigned Version is not configured to be used_as_backlog?, we can keep the Version association. However, if the Version poses as backlog, we must remove the association, or else the Story would exist in an Agile::Sprint and a Sprint at the same time.

The feature flag is not checked within the stories controller. As you can only move a Story to an already existing Agile::Sprint, I think this is acceptable. Having the feature enabled is required to create an Agile::Sprint. Modifying one should be allowed even without the flag.

Caveats

Work Packages do not consider and update their position with Agile::Sprint in mind. Hence, you can drag and drop to an Agile::Sprint - but once the Work Package is in an Agile::Sprint, the position will not work as expected. This issue is out of scope for the current PR and will be handled in WP 73093

[dombesz]

Overall works as expected, I found a few code structure related issues and one permission check issue, nothing serious.