added student number to Users and its upgrade script
added student numbers to user add/edit/view and classlist view

- I've added validation for unique user identifiers (username,
display name, and student number) into the users add/edit
api functions.
- There is nothing to extravagant for adding users. However for users
edit, I can only assign the new values to the existing user model
if it passes the uniqueness validation. If I don't check before assigning,
the api will return a 500 error for an IntegrityError (this is thrown
before I have even committed the object)
Major refactoring for class import

- included student number in list of user information accepted
- updates existing users
	- will not change system type - in case an instructor is
	included
	- will not change password - even if one is given
	- will not change field if a value is not given
- if empty file is given, all students would be unenroled
- new error messages for student numbers
	- if one is given, it has to be unique in the system and in
	the file.
implemented the ability to import groups by student numbers

- like class import, if an empty file is uploaded, all members will
be unenroled from their groups.