Require type for users

1. require user_type (== student | == teacher) for all new users
2. add a script to fix existing users so that users who are acting as teachers (but not user_type: teacher) are teachers and users who are not acting as teachers and do not have a user_type yet are students

Later:
require that all followers/sections belong to teachers
restrict access to teacher ui to users where user_type == teacher
etc