[bug 1078326] Change question -> {product,topic} to Foreign Key

This includes a schematic migration that needs to run before south to remove a conflicting product_id column that was in the db but not the model yet.

The rest should be mostly straightforward. I renamed some things to try to make some things less ambiguous and easier to understand.

This is just missing the migration to drop the M2M tables.

r? (@mythmon)