Create prerequisite for courses

[karbassi]

Allow Courses to have prerequisites.

For example, JS 1 has no other course prerequisites. However, JS 2 has a prerequisite of JS 1. JS 3 has JS 2, and so on.

To Do

• [x] Modify the Course model to allow for specifying a classes prerequisites
• [x] When a guardian goes to register a student for a session, allow the registration only if the student has met the class prerequisites, which should use the criteria:
  • Student has at least one order for an active session that was a prerequisite class
  • And that session was in the past
  • And there is a non null check-in date
• [ ] Update the programs template, removing hardcoded prerequisites and replacing with the prerequisites defined via the data model
  • ./website/weallcode/templates/weallcode/programs.html
• [x] Display the required prerequisites in the class template
  • ./website/weallcode/templates/weallcode/snippets/class.html

Research

• http://www.softwaregems.com.au/Documents/Student%20Resolutions/Swartz%20DM.pdf
• https://stackoverflow.com/a/4150138
• https://github.com/django-danceschool/django-danceschool/blob/master/danceschool/prerequisites/models.py (this is crazy details and overkill for what we need)