We now allow outgoing webhooks to provide us a
"content" field, which is probably a more guessable
name than "response_string", particularly for folks
that use our other bot-related APIs. And we don't
modify content as we do response_string, i.e. no
"Success!" prefix.
If we're not too concerned about backward compatibility,
we can do a subsequent commit that makes "content"
and "response_string" true synonyms and get rid of
the "Success!" prefix, which was probably accidental
to begin with.