added join migration and revised other migrations and models

I think this is the way to add a join table. What I think is that the both the students and the courses belong to the location. A student can take many courses and a course can be taken by many students. So the join table just needs to relate the 2 tables together.

I forgot that the join table does not require an id unless it is a rich join in which case this is not. I will make that change and push a new file.