Depends on D19928. See https://discourse.phabricator-community.org/t/firehose-webhook-not-working-with-self-hosted-requestbin-instance/2240/.
Currently, we report "hook" and "silent", which are raw internal codes.
Instead, report human-readable labels so the user gets a better hint about what's going on ("In Silent Mode").
Also, render a "hey, you're in silent mode so none of this will work" reminder banner in this UI.
