Make second user admin

In seeds.rb we create the first user which is 'User deleted', so the first actual user which we need to become an admin is the second user of the database.

https://github.com/openSUSE/osem/issues/456
I know what it does. I'm just saying that this is so crucial that you should check all the possibilities. For instance, what if the admin changed seeds.rb. What if there is already a user in the DB (that isn't admin) for whatever reasons. Please make sure this is bulletproof. Just checking "is second" is a bit too less for this very crucial method...