move user management code from sync server to common code

So it can be called from:

• HTTP server and
• sync server.

Methods:

• createUser
• changeUser
• forgotPassword
• loginWithPassword
• loginWithToken

Store mail formats and domain config in /[domain]/userconfig in streaming store, with fields:

• newUser
• newLogin
• passwordReset
• passwordChanged
• password reset secret key
• password reset public key
• from mail
• from name
• display name
• (alternate domains/names)

(should we store this under /userconfig/domain; this way all the configs can be loaded at once)

Various:

• Don't forget about the reset password function implemented in Lua (to move that to C++)