Fix performance regression

Remove course_from_id
don't fetch whole course when the root will do
don't fetch even the root if the id will do

@chrisndodge @dianakhuang @sarina @nasthagiri some subset of you, please review

I believe @cpennington  added a mixin class to deal with this but it's not working quite right. I don't want you to put in ignores, because that's a bad pattern (SSCK does have run, but CK doesn't, so it's a valid warning in some sections of the code base).