New option to allow coaches to add tasks and solutions

Tasks are created in the course editor. In some situations it is desired that the course author only creates the tasks course element, however the creation of the tasks itself should be up to the coach.

Since the tasks "database" is currently part of the course structure data model this will require some refactoring. The task descriptions will be moved to some place that is independent of the course node configuration and the publishing process.

Tasks

• Move task datamodel from course node configuration to independent place (DB or filesystem)
• Add option to course node to enable task management for coaches
• Implement task management for coaches and owners on course node run view, for coaches only when enabled in course node configuration
• Allow publishing without tasks, add message to run view if no tasks are available
• Implement course export/import, same behavior as today
• Implement course copy, same behavior as today
• Implement course node copy, same behavior as today