Backward compatibility of airborn with account data

Every Airborn account has an integer account_version which it has access to after logging in and which encodes some information about the inner workings of Airborn at the time the account was created. Client and server side code can use this variable to implement backwards compatibility. New accounts are assigned the latest version. The versions are:

• 3: The latest version.
• 2: Accounts with this version may have the /key and /hmac files encrypted directly with their password. (Exists only on airborn-dev.)
• 1: Accounts with this version may have any file encrypted directly with their password. (Exists only on airborn-dev.)