Add API_PUBLIC_URL environment variable for public API access

- Added API_PUBLIC_URL to deploy-config and manifest.yml
- Updated app config to load API_PUBLIC_URL from environment
- Used API_PUBLIC_URL in CSP configuration for connect-src

app/config.py

@@ -12,6 +12,8 @@ class Config(object):
     NOTIFY_APP_NAME = "admin"
     NOTIFY_ENVIRONMENT = getenv("NOTIFY_ENVIRONMENT", "development")
     API_HOST_NAME = getenv("API_HOST_NAME", "localhost")
+    API_PUBLIC_URL = getenv("API_PUBLIC_URL", "localhost")
+
     ADMIN_BASE_URL = getenv("ADMIN_BASE_URL", "http://localhost:6012")
     HEADER_COLOUR = (
         "#81878b"  # mix(govuk-colour("dark-grey"), govuk-colour("mid-grey"))