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.
How about a more generic structure like 1) Intro: We've encountered a technical problem with (video, assignment name, etc.). 2) Status: We are working to resolve it OR it has been fixed. 3) Action taken, if needed, for example: We've extended the deadline for or rescored this assignment so that it won't affect your grade. 4) Thanks for your patience, and we look forward to continuing the course with you.