Add 'encryption' config options group

We have to save the value per account and contact, because otherwise
it could lead to problems if a jid is added to more than one account.

As key this uses a string in the form of 'Account-BareJid'.

The config option group 'contacts' is not used because there are values
like 'speller_language' that make more sense to set on a Jid basis, rather than a
Account-Jid basis.

Treat an empty config value or no config value as 'disabled', instead
of writing a config value of 'disabled'. This has the benefit that we
dont have to write config values for contacts were we dont use encryption.