Add role to primary key in Credit

1. Fixed previous migration script(2e3e3a9a625) for altering only column name.
2. Added 'unknown' element for `Role`. Because, If a column that has a `Role` type and is used for composing the primary key, you wish the column has a default value. (cannot set to null)
3. Added  missing `Role` to `__all__`.
4. Added migration script for adding 'unknown' to `Role`