Test coverage and refactor

Just did a bit of refactoring (fat models, skinny controller style) and fixed the capybara tests plus added more coverage for funs...
You could make this (or call this from) `def receivers=()` and let the constructor set this value. I'm pretty certain that would work (it's been a while). You would just need to make sure the form was nesting it under email: `params[:email][:receivers]`. The bonus is that the controller would look like this:

```
def create
  @email = Email.create(email_create_params)
end
```