Enhancement #520 - hide marks on evaluations

- This enhancement allows the evaluation marks to be hidden/shown to
  users.
- The marks can be hidden for mixed evaluations and rubrics.
- The marks can be hidden by checking the checkbox that appears.
- Upgraded the database to version 7.
- This version adds a single column to 'rubrics_critereas' and
  'mixeval_questions' to signify whether or not to hide/show marks.