At the moment it's a little unclear why a signup fails if send grid hasn't been configured. We should replace the generic 500 error with something that makes it clear signups don't work unless sendgrid is configured.
[x] Add documentation page that explains why send grid needs to be configured and how to do so.
[x] Add a more specific error message to the signup endpoint if send grid isn't configured.
At the moment it's a little unclear why a signup fails if send grid hasn't been configured. We should replace the generic 500 error with something that makes it clear signups don't work unless sendgrid is configured.