Migration should have `on_delete: :delete_all` for user reference

[danschultzer]

The Ecto migration mix generator has been lifted from the Ecto library where on_delete: :nothing is unfortunately hardcoded:

https://github.com/danschultzer/pow/blob/v1.0.20/lib/pow/extension/ecto/schema/migration.ex#L14

In the case of user identities it really should be on_delete: :delete_all as recommended in the Ecto docs. I'll have to rework the mix generator to permit defaults being passed along with associations.