Sjang92/enroll email squashed

@jrbl @nparlante 

Enrollment Email Feature

From Jane's feature description:

"Students are sometimes confused by the fact that they don't get an email to let them know when they've enrolled in a course.  This feature would have the system send an introductory email that will confirm enrollment.  There will be variants of that email for when they enroll: eg. before the start date or not."



I dislike how many distinct stanzas of imports there are here, and they don't seem to make a lot of sense in their groupings. Please put these imports at the very top, and move the namedtuple import to the same stanza. And alphabetize, so it goes json / namedtuple / timezone.