Use validation of recipients from utils

This was added to utils in 5914da74f1

This means that:
- we are doing the exact same validation in the API and admin app
- we are actually validating phone numbers for the correct format (hence all the
  changes to the tests)

tests/app/conftest.py

@@ -277,7 +277,7 @@ def sample_notification(notify_db,
     if to_field:
         to = to_field
     else:
-        to = '+44709123456'
+        to = '+447700900855'
 
     data = {
         'id': notification_id,