Extracts creation of alchemy_roles column and index to own migration.

Since the user model is moved out of the Alchemy core, the roles column is namespaced with alchemy.
The first migration that creates the alchemy_users table will only run when it does not exist already. When upgrading an app using an Alchemy version smaller than 3.0 it comes with that table, so the migration will not run, but the existing roles column has be be updated anyway. That's the reason why we moved that column change into a separate migration.