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.
excellent idea! do you have a template already? if not, perhaps we can make a new DOC story to add that in the next sprint, rather than holding up release of this chapter while I research and write it. Let me know if that strategy is ok.