Reduced Credit Date (develop)

This makes the reduced credit feature use individual dates for sets.

- Reduced credit now begins after a specified date (which is in between the open date and the due date).  This date can vary from set to set, and from student to student.  
- The reduced credit feature is now turned on and off in course config.  If its off then options relating to it are not shown.
- The reduced credit period in course config is now a default period and is used to define a default reduced credit date.  I can be overridden in the set detail page.
- The reduced credit percentage is now a percentage select.
- The reduced credit date is kept in the database and this commit will require a db upgrade.
- The reduced credit date upgrade has an associated pg pull.  In particular the way the reduced credit graders work had to be changed since the reduced credit logic was built into them.  This may break custom graders that use the old reduced credit logic.

- I also added an option in the config area which enables or disables conditional release, and will hide the options relating to it.
- I upgraded conditional release so it cannot be "hacked" using the url.