Encrypt passwords for mail accounts

[tforgione]

We need to be able to access the passwords for the SMTP / IMAP accounts, since we will need to send them to the SMTP / IMAP servers to authenticate, therefore, hashing the passwords is not sufficient.

I think a good thing could be is to use the user's chouette password to compute a private key to encrypt its SMTP / IMAP passwords so that:

• when the user connects, we decrypt its accounts passwords and store them somewhere (maybe in the session)
• no passwords are not stored in clear in the database
• if the database leaks, the attacker can't retrieve the chouette passwords of the users because they're hashed, and they can't retrieve the SMTP / IMAP passwords because they would need the chouette password