Support adding students to a cohort

TNL-163
I guess we should decide which is clearer as I've been using the ```interpolate``` fuction rather than ```_.template```. The benefit of your approach is that the templates are easier to read, rather than the ugly ```%(numMoved)s``` way of specifying values. I don't like that your version has that very confusing interpolate parameter. Any preference? Maybe we should move this into TemplateHelpers so that we all do it the same way.