Course prerequisites

[rgroves]

With this PR:

• Courses can be assigned prerequisites
• Registration for a session will only be allowed if a student has the necessary prerequisites
• If a student still needs to take one or more of the prerequisites, buttons will be displayed for each prerequisite
  • where an upcoming session is scheduled for a prerequisite, its button will be active
• The upcoming classes list on the Programs page will list prerequisites associated with the class for the session

This partially addresses #816. The outstanding item needed to close this out is related to the Courses list on the Programs page, will comment on the issue with details.

[rgroves]

@karbassi I have added the prerequisites to the class view templates for the guardian and public views. I didn't think it made sense for the mentor view—don't want to have someone think they can't mentor a class before mentoring for a prerequisite class. If you don't agree with this let me know and I'll add it to the mentor's class view template as well.