Option to encrypt the addressbook

[allo-]

Seems like the addressbook database (in sqlite format) does not use any encryption. Other private things seem to be encrypted with the login password.

Maybe the database should be split in one file per user (to enable easy deletion of old users, see #1051) and then encrypted with the user's password (or an additional password chosen by the user, to prevent problems when the password is changed on the mailserver without changing stuff in rainloop).