Adding grad students

[bjmckenz]

Part one:

• students are undergrad or grad
• can be edited by admin
• "type" shows in their header for login
• shows up where class level used to
• filtering permitted in user/student lists.

(part 2 will finish job with class reg restrictions)

Part of #187.

[bjmckenz]

@liannadevries09 can you take a look at this part, testing is described in #187. Would like this to be solid before committing next part. Thanks!

[oceancypress]

Tested, looks good