fixes #6873 - ensure hidden 'owner' is populated on user role

Also re-order seeding such that the anonymous role is added to seeded users.

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I've filed http://projects.theforeman.org/issues/6878 to consider moving away from default scopes.  Here the default (hidden) scope on User is causing a problem with the UserRole association, as owner_id is populated, but owner is nil.

If you do the 1.4 to 1.6 upgrade (see #1639 description) without the user.rb change but with the seed reorder, you'll see the error clearly, as the anonymous role will be present and get added to the hidden user.
UserRole ends up with a nil `owner` attribute using the `<<` operator.  owner_id is populated on the UserRole, but I think because of the hidden user in the default scope the owner (User object) itself isn't found.  This is a workaround to ensure that owner is populated in the user role.