Add campaign UTM links and piha.li shortener as a core helper.

Keep mint/UTM in always-on core so email_sms and directmail stay optional, and brand utm_source from SITE_NAME (or UTM_SOURCE) instead of a hardcoded client name.
Closes #3

scripts/validate-env.sh

@@ -53,6 +53,14 @@ if _true "${FEATURE_DIRECT_MAIL:-}"; then
     PCM_RETURN_ADDRESS
   )
 fi
+if _true "${FEATURE_EMAIL_SMS:-}" || _true "${FEATURE_DIRECT_MAIL:-}"; then
+  if [[ "$DJANGO_ENV" == "prod" || "$DJANGO_ENV" == "beta" ]]; then
+    required_vars+=(
+      SHORTENER_BASE_URL
+      SHORTENER_API_TOKEN
+    )
+  fi
+fi
 if _true "${FEATURE_PAYMENTS:-}"; then
   required_vars+=(
     STRIPE_SECRET_KEY