DO NOT MERGE bulk email work for discussion

@cpennington @dianakhuang Could you guys take a look at the tests I have written for the bulk email interface so far? They just check that the email link will display in the instructor dashboard if the settings flag is enabled, otherwise not. Are there other tests you want to see for the interface? It's still a work in progress, but I'd appreciate your input on the interface tests for now. Models.py and tasks.py are still being worked on.