Add email opt-out system

Add email opt-out system

We're going to start generating emails on patchwork updates, so firstly
allow people to opt-out of all patchwork communications.

We do this with a 'mail settings' interface, allowing non-registered
users to set preferences on their email address. Logged-in users can do
this through the user profile view.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>