Support adding cohorts from the instructor dashboard

TNL-162
I don't like the ```toBe(0)``` syntax because it fails with '0 is not 1' so you can't see what the true error is in the logged message. I also think that checking for existence is cleaner because that's what the test really wants to be. 

However, you are right that the method doesn't seem to work as I thought (which is a problem as I've used this quite a lot in Studio code). It tests whether it exists 'in or out of the DOM', so just because it has been removed doesn't mean that it doesn't exist.

https://github.com/velesin/jasmine-jquery/issues/64

There are lots of interesting methods that we should be using because they give better error messages. I'll work out which one it should be.