Make subject_assignment! method

@wvanbergen @pseudomuto 
/cc @richgilbank 

I'm not sure if you're going to want to accept this one, but I figured I'd see.

I noticed that we were calling the combo of `subject_assignment` then `store_assignment` twice in the rake file. It's also a combo that we're going to want in Shopify, when we're manually assigning groups that were made in signup.