Minor style changes

* Rename .add to .image-control; lots of things get added, so "add" is
    pretty ambiguous
* add a little space between image-control panes
Use an in-memory celery broker in tests

Sending tasks to a real broker causes a bunch of noisy errors in the dev
worker, since of course the data in the test db isn't visible...
Error emails in the Celery worker

Also, rename the email envvar names to not be sendgrid-specific