Implement activity locking (better name might be Activity Prerequisites)

For each activity module, add two things:

1. Ability to define a criteria when when the module is finished by a student. For example, a pass mark on a quiz, etc.

2. Ability to lock the activity based on some other activity being finished.

eg this will allow activities to be chained, so that for example, a forum isn't available until a quiz has been passed.