Warn the user if deploying something else than master in PROD

[alexduf]

As far as I'm aware we are mostly deploying the master branch to PROD, and very rarely branches (except emergency fixes and such)

It would be wise to have a big warning if you were about to deploy anything else than master to the PROD environment.

Even though it never happen to me (yet), I did scare myself a couple of time by mechanically picking "PROD" in the dropdown.