Send user feedback through Briar
#124 (closed) covered user feedback sent over Tor. This issue covers the extension of sending it through Briar.
The app should post feedback to a feedback channel.
Messages in the feedback channel should be synced with contacts, but no further. We'll need to add a special contact that's only reachable via Tor for delivering feedback to the developers. To prevent the special contact from being a single point where Briar users can be enumerated, it should only be contacted when there's feedback to deliver.
Users should be able to opt out of delivering feedback for their contacts.