Filter out the user's own uploads and favorites from their
recommendations.
Note that in most cases a user's top-N recommendations will be things
they've already favorited. If a user has 10,000 favorites, most of their
top 10,000 recommendations will be their own favorites, so we have to
generate a little more than 10,000 recommendations to be sure they won't
all be filtered out.
In other words, the more favorites a user has, the more recommendations
we have to generate. The upper bound is clamped to 50,000 for
performance reasons. If a user has more favorites than this we may not
be able to find any recommendations for them.
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