Order contributors

fix #411, fix #412, fix #405

This PR adds an order field for contributions (thus a migration is required when applying it).
When editing a course as staff member or editor, the contributions can now be sorted via drag and drop. Students then see the questionnaires in the order in which they have been defined.

This also fixes #405 by adding the ```<thead>``` tag in the contributor view.
```form_group[contributor].items()``` returns the form group for the specified contributor.
Its first element is a tuple consisting of another tuple and an object ```((Contribution, Questionnaire), QuestionForm)```, so the contribution can be retrieved by ```form_group[contributor].items()[0][0][0]```