Mint campaign UTM links through the piha.li shortener.

Closes #9.

Auto-insert tracked homepage links in campaign compose, register them with
url_shortening_service, and put short piha.li URLs in SMS, email hrefs, and
postcard QR codes when SHORTENER_* is configured.

scripts/validate-env.sh

@@ -28,6 +28,8 @@ if [[ "$DJANGO_ENV" == "prod" || "$DJANGO_ENV" == "beta" ]]; then
     RECAPTCHA_PRIVATE_KEY
     EMAIL_HOST_USER
     EMAIL_HOST_PASSWORD
+    SHORTENER_BASE_URL
+    SHORTENER_API_TOKEN
   )
 fi