Add GOV.UK unicorn config
Add GOV.UK unicorn config

As per the instructions [1] for the upgrade of govuk_app_config from
v1.2.1 to v1.3.0.

[1]:
https://github.com/alphagov/govuk_app_config/blob/5220b89da3be24429656e69f0c0f90bef11abdd4/CHANGELOG.md#130

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager