Migration for role_coursenum deprecated roles

@cpennington please review. I think we should add a modulestore.find_course(pattern) command to do searching over course ids, but I didn't want to pile on too much on this pr.

@dianakhuang please test on dev (or tell @feanil when to test it). It should convert most of the roles in Feanil's gist of missing conversions. We'll need to check by looking at
```
select * from student_courseaccessrole;
```
to see if all the possible translations from his gist (Feanil, can you post the gist here?) are in that table.