Remove deprecated debug toolbar settings

INTERCEPT_REDIRECTS throws a DeprecationWarning. I think the default
config for the debug toolbar is sane, so just removed it.
Remove raw SQL from oscar_calculate_scores

The Django ORM is perfectly capable. F() expressions FTW!

Unfortunately, this exposed a bug in the raw SQL, affecting both SQLite
and PostgreSQL. I'm somewhat confident that previously, only num_purchases
was divided by the total weight.

Will write docs later so I can link to this commit.

Issue #1033 is related.