Chatmail: Automatic account creation

[s0ph0s-dog]

This is part of a series of issues tracking chatmail support in Mox.

Chatmail servers allow users to sign up by choosing a username and password and connecting to the server with that username and password. If the username is not already taken, the server creates the account with the provided password. This makes account setup as easy as possible.

• Add a configuration option (AutoregistrationEnabled default true) that allows administrators to turn off automatic account creation (as an emergency stop button in case of abuse, or just because they want to)
• Modify OpenEmailAuth() to create the account if it does not exist, if the configuration permits, and if the client chooses an authentication method that transmits the password to the server
• Add tests to verify that automatic account creation only happens when the option is enabled