Certs <-- has and belongs to many --> Courses

[masukomi]

After speaking with @kgf it was decided that there should be a HABTM association between courses and certs, because a course can convey multiple certs.

This PR adds that functionality.

On a related note: I don't really understand why a you associated a cert with courses instead of a course with certs. I changed the cert form to support multiple courses. It doesn't feel quite right but i believe that it is, because there are multiple courses that could, for example, convey the same cert. For example the long First Responder / First Aid course includes CPR training but you could also have a short CPR course that conveyed the same CPR cert.

That being said, it feels wrong that you can't choose certs when creating / editing a course.

[kgf]

Amazingly enough, I am open to renaming the models if it would make it easier for people to understand.

There is a person. A certification is that person completing some set of hurdles, such as taking a class and a test. An example of this would be a drivers license. They studied the book and then took the driving test. They are now licensed/certified to drive a car. This certification will expire. It is for one course.
A long first aid course with CPR gives two certs; First aid and CPR. It needs to work this way, because they expire on different timelines.
You aree quite correct that it does get complicated.

[kgf]

@masukomi Would you be willing to freshen up this PR ? It did not pass the tests...