4 characters check on verif code and string errors
4 characters check on verif code and string errors

• Add check to ensure verification code sent by SMS is exactly 4 characters long
• Change error data from JSON to simple string error code
• Change `hello` Cloud Function to `slove`
• Add easy switch for Twilio to go from test to live

julienpa/slove-server,julienpa/slove-server