[hotfix] handle new signal verification code format
- SYMPTOM: registering new numbers was timing out on the verify step
- CAUSE: signal changed its verification message format from "Your Signal verification code: 123-456" to "SIGNAL: Your code is: 123-456"
- FIX: relax
signal.parseVerificationCode
to accept any 6 digit code prefixed by a colon (so that signal can change the format again and it won't break our registration logic)