Publishing API now expects a 'publishing-app' field

This expectation was added after short-url-manager was built, but
the change was causing redirect registration to fail since the
API change

(https://www.agileplannerapp.com/boards/105200/cards/6075)

I wanted to catch the specific exception and log it, to something like errbit (so that debugging API errors can be done more easily next time), but I wasn't sure how with Errbit.
```
rescue GdsApi::HTTPErrorResponse => exception
  log(exception)
  errors.add(:base, "An error posting to the publishing API prevented this redirect from being created.")
  false # Do not continue to save
end
```