New course element "check list"

A new course element "check list" shall be implemented to support different didactical concepts like:

• to-do list
• control lists
• attendance list
• preparation lists
• self controlled learning/activities

The course element has the following features:

• definition of checkboxes by author
• optional file attachments per checkbox
• optional score per checkbox (rubrik like assessment)
• learner / author editable checkboxes
• optional due date (checkboxes blocked for learners after that date)
• different checkbox labels for different usage scenarios
• recalculation of efficiency statements when publishing changes in the scoring model
• group filter in view
• printviews for all users, per group, per checkbox

For more detailed spec, see attachment. See also OO-937.

Note:
In the code already exists a course element with similar functionality. However, this code has many issues (not thread save, wrong storage usage, leaking to other courses when copying, overall poor code quality). Thus we decided to not reuse that code but rather start from scratch. The course elements will be marked as alternatives for semi-automatic migration of existing elements.