Split serializers into multiple files.

Seems strange now, but it'll be clean when we have to add different
serializers for read/write and validation methods for each field.
Add CreateUserSerializer.

This adds a new serializer for creating Users, which contains the
password field and validation methods for username and password. It
also adds the UserListPostTestCase. I don't think other serializers
(other than perhaps school) will need much modification, but this
can serve as an example.