Add the ability for Microsites to be able to send appropriately branded ...

...emails rather than the default system installed version

fix tests. Update Admin views/forms.

enforce uniqueness constraint in the Admin ModelForm submissions
style foul (with '\').  I think the following style is cleaner
```
email = CourseEmail.create(
    course_id,
    request.user,
    ...
    from_add=from_addr
)
```