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."



Following Ali's initial front-end implementation - show and hide elems according to the enable-enrollment-email flag