HG-236: Change `production` to `prod`

The concept in Wikia is to use `prod` for production environment. The aim of this PR is to unify the environment name in Mercury by switching from `production` to `prod`.

There are more changes to be made in external services but this setup is safe to work with both `prod` and `production` until these changes are made. When this is done, the two `TODO` comments must be addressed, which will break the backwards compatibility.
hmm sorry I know it was me
but how about changing
`gulp --type`
to
`gulp --env`
?