mattdrayer/api-roles-opaque-keys: Fixed unhandled DoesNotExist exception

@chrisndodge @martynjames -- here's an initial attempt at a fix for ticket 1974 -- I've added support for converting a new-style id back to the old-style in the case of a failed lookup.  Check out the test logic -- I'm creating a course that has no corresponding forum moderator role, which helped to trigger the error condition.  I'm a little unsure of what to do if the secondary lookup for the forum role fails -- right now i"m just shrugging my shoulders and doing a 'pass'.
I'm wondering if the exception case is going to be the typical case.

@martynjames was setting this role failing always, or just on some courses? @antoviaque one thing I'm concern is whether on the cs_comment_service side, the assumption is that it is ALWAYS legacy course_id format. I wouldn't want us to get into a state where some courses have this course_id in the roles table one way and in other courses, it is another. Mainly because I'm not sure if the "other side" of the forums is expecting things in opaque-key format...

If we see cases in testing where forums don't appear to have a TA when the are supposed to, that would indicate an impedance mismatch.