Ahodges/documentation/doc368

includes both DOC-368 and DOC-229
- addressed the incorrect information on issuing certificates at the end of checking_student_progress.rst (DOC-368). @WatsonEmily , Alison K filed this issue for MIT.
- moved all info about the student progress page so that it is delivered together in course_grades.rst.
- deleted out-of-date, redundant info on instructor view of the student progress page from checking_student_progress.rst 
- moved all info on student answers out of course_grades.rst and into a new course_answers.rst
- applied gray border and sizing recommendations from the edX 101 Image Style Guide to images in these rst files
- change log
@srpearce , @mhoeber please sanity check.