Support multiple-listed classes

[nathanmsmith]

The same course/section can be listed by multiple subject areas. Right now, Hotseat can't handle this: we consider each course to be a completely separate course. Here are some initial thoughts here:

Thoughts

DB schema:

1. The multiple-listed status could change from term to term, but each row in our courses table refers to the same class across different quarters. As a result, we may want to maintain the many-to-many relationship of multiple-listing in a table separate from courses, with term as a constraint.

User-visible pages:

1. We could try to combine multiple-listed classes into a single page, while keeping the current DB schema for courses and sections. But it can be hard to determine the "canonical" subject area for a class – which could actually be different based on the term and section.
2. Alternatively, we could keep separate pages for multiple-listed classes (and keep the current DB schema), but provide links to the other multiple-listed classes and display reviews for the other equivalent courses on the current page.

Test cases

A most extreme example would be LIFESCI 192A:

More normal examples include COM SCI M51A / EC ENGR M16.