Add release to story map when epic is added to step

Currently the release doesn't show up on a story map until there is at least one story added.

Epics have releases as well, but currently when adding an epic, the release doesn't show up in the story map.

For planning purposes, the epics may not have been broken down to full stories yet, but we know when we want to do it. Without this feature, the release would not be visualized in a story map as there is no story assigned yet.