feature: display due dates for stories in backlog sprints

It could be useful to see story due dates inside backlog sprints. Therefore I added such possibility. Due date is shown next to the story status. If story has no due date set, nothing is shown. You can enable/disable this functionality on backlogs settings page (with a checkbox just below `show project name in backlog`).

Sorry for messing with withespaces in few unintended places - that's because of my editor configuration.