Move #invoke_callbacks to Admin::BaseController.

Add callback hooks for Admin::PaymentsController#create.

Allows extending payment controller for new payment methods such as gift vouchers:
See https://github.com/spree-contrib/spree_vouchers/commit/def90bf508c383ffa3a56786aa570ba7b8677618#commitcomment-5709993 for the reasoning behind this.

I'm receiving a couple inconsistent payment spec failures locally, but travis is passing... I suspect it's some poltergeist voodoo that perhaps @jhawthorn can help me with.