Perform slug validation before checking for uniqueness

[namxam]

Performing the uniqueness check before slug is sanitised allows to users to create multiple leagues with the same slug. That’s reaaaallly bad!

[stephanpavlovic]

sounds like a reaaaally bad idea

[yoyostile]

Erm, guys. The build is failing: https://travis-ci.org/railslove/kickerapp/builds/182008854

Besides that: I think this is a great brainteaser for @DonSchado 's https://github.com/DonSchado/broken-app :)