Four enhancements to support the Spring 2018 term (needs to be ready by end of December)

1. When students submit the response (pdf) to an assessment , they should be able to review before submitting it, to make sure they are not submitting the wrong file.
2. Implement a graceperiod, which is some grace period the course team allows student to submit their answer, after the deadline shown to students.
3. Course staff to be able to download all student submissions in one click, as a zip file. Now, they have to download answer files one by one.
4. After a student submit the response (usually a pdf file), the answer (a pdf file) should be revealed to her. Currently the answer is revealed only after deadline.