Adding course section dropbox page

Change list:
- removed unnecessary code from dropbox (Dan Isler will update the index function)
- **dropbox controller will now use your homework table in db1.py for student homework submissions**
- added folder table to db1.py (need to organize assignments within a class section)
- removed folder table in db_submissions.py 
- added course section reference to table
- added opening date to allow professor to create homeworks, but not allow students to access them yet
- added course_dropbox function for displaying assignments for a course section to the user (also provides links to other functions within the dropbox controller; submit, etc.)
- added course_dropbox view