Encrypted crash reports submitted via Tor

If the user chooses to report a crash, the crash reporter should encrypt the crash report with the developers' public key and store it on the disk until the app is re-launched. The app should import the encrypted report, post it to a crash report channel and delete it from the disk.

Messages in the crash report 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 reports 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 are crash reports to deliver.

Users should be able to opt out of sending crash reports and opt out of delivering crash reports for their contacts.