Test cohort selector in NewPostView

@jimabramson @andy-armstrong @cahrens @dan-f 
@gwprice and I hashed this out in a private discussion, and I agreed with him that the id approach makes sense here. I hold an extreme position that I never use ids, so even seeming them in tests makes me question the correctness of the code. In our discussion, I realized that in my test I don't pass an empty div as the ```el:``` to a view, but instead use JQuery's default behavior of creating an empty div for you if one isn't passed in. So this isn't an idiom that I've ever used.

Also, when we move all of our templates into their own static files, there won't be a natural central place to put this empty div like there is now. However, we can cross that bridge when we come to it.