Use GOVUK_ENVIRONMENT env var to set styling

[theseanything]

There are a set number of apps that use this gem. They all configure the environment_style and environment_label with the exactly same configuration. This moves the repeated configuration into the this gem as standard.

e.g. https://github.com/alphagov/support/blob/main/config/initializers/govuk_admin_template.rb

GovukAdminTemplate.environment_label = ENV.fetch("GOVUK_ENVIRONMENT", "development").titleize
GovukAdminTemplate.environment_style = ENV["GOVUK_ENVIRONMENT"] == "production" ? "production" : "preview"

This will have no effect on the behaviour of the dependent apps, and allow us to remove repeated config.

The only apps that use this gem are:

• Publisher
• Maslow
• Support
• Specialist publisher
• Short url manager
• Transition
• Service manual publisher
• Contacts admin
• Search admin
• Content tagger

And all have the same configuration.

[ChrisBAshton]

LGTM 👍