Overhaul the user data structure to use a class

No more arrays of lines in a file, and no more passing `username`
and `info` into the template! I also added an `admin` group instead
of having an `admins` file. Administrators now have a different
navbar with 'grades' and 'submit' replaced with 'admin'. `/emails`
has been renamed `/users`, although this is pretty superficial
and could easily be reverted.
Hide non-admins from the `/admin/grades` list
Include requires_login in requires_admin