Task enhancements for project module

[ri0t]

• [ ] Selected tasks should be able to expire gracefully
  • [ ] to a custom taskgroup
  • [ ] soon expiring should be well visible
• [ ] Finalize the referencing system
• [ ] Create calendar entry from tasks with a duration -> for Gantt etc #219
• [ ] Finalize github importer (mentioned in #219, as well)
• [ ] Notifications about expiring tasks (blocked by: #328)