remove references to address-book (and maybe replace with Contacts)

[taggart]

All the pages in email/ have an entry in the FAQ similar to:

* [[Is there an address book? => address-book]]

That is old from the previous webmail clients and the page was removed but not the references. Now in roundcube there is a "Contacts" interface that serves the same purpose.
We need to either drop the old references, or make a new Contacts page and point to it instead.

When amber rebuild'ing currently this produces errors for all the references like:

warning: dead link to `address-book` from page `/en/email`