Add submissions cache key to syllabus.
Add submissions cache key to syllabus.

Added the submissiosn cache key to the syllabus parital cache key.

Closes #528.

peterkshultz/gradecraft-development,UM-USElab/gradecraft-development,UM-USElab/gradecraft-development,zqian/gradecraft-development,peterkshultz/gradecraft-development,zqian/gradecraft-development,mkoon/gradecraft-development,mkoon/gradecraft-development,mkoon/gradecraft-development,zqian/gradecraft-development,UM-USElab/gradecraft-development,peterkshultz/gradecraft-development,UM-USElab/gradecraft-development