Adds sample student communications (email msgs)

@mmacfarlane , @WatsonEmily , please review and provide many opinions on what to add/delete/change. An html version is available here: http://draft-student-communications.readthedocs.org/en/latest/releasing_course/course_launching.html

@mhoeber , @srpearce please copy edit and proofread.
> "and received their role assignments"