no more db:migrate, just db:setup

this PR moves seed data that was previously in the migrations to the "redmine:load_default_data" rake task. you can/should now use "db:schema:load" followed by "redmine:load_default_data". also rails' canonical "db:seed" task got enhanced with "redmine:load_default_data" so you can use that one, too. taking both things together you can now just run "db:setup" when you bootstrap your openproject instance.