Display a better environment label
Display a better environment label

With these changes:
* a label is included in emails from staging, because Plek's parent_domain is
the same in production and staging (production.alphagov.co.uk)
* we use the same label we show users in the app to distinguish environments
* the behaviour won't change (break) when we move to the service domain for our
apps: production.alphagov.co.uk => publishing.service.gov.uk

ggoral/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,robinwhittleton/whitehall,alphagov/whitehall,hotvulcan/whitehall,ggoral/whitehall,hotvulcan/whitehall,askl56/whitehall,YOTOV-LIMITED/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,askl56/whitehall,askl56/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,alphagov/whitehall,alphagov/whitehall,robinwhittleton/whitehall,ggoral/whitehall