Allow moving sprints on Scrum Boards by drag-drop.

In our conventions, each sprint targets a specific feature, that we work to implement. Sometimes re-prioritization occurs. It would be nice if we could just change the order of already planned sprints quickly.

One can, of course, move the contents of sprints already, but it soon becomes cumbersome if you need to move items from sprint A into B and from B into C and from C into A etc. It would be simply more convenient to allow re-ordering the sprints themselves.