Release backlogs

The new Planning board supports a single backlog.

For the purposes of release planning, one or more release backlogs can be very useful.

Release backlogs can almost be supported in the current board simply by naming a sprint after a release. During planning on the Planning board, we can drag stories among the product backlog, the release backlogs, and sprints.

The only thing that is missing is this - we would like to create new sprints for a release above its release backlog. However, the current Planning board does not allow the creation of a new sprint above another, nor does it allow sprints to be reordered. Consequently, to create a new sprint for a release, we're reduced to: create a new sprint above the product backlog, and then manually shift all the stories in each sprint to the sprint below and rename each sprint.

Please consider supporting release backlogs in addition to the product backlog.