When a user changes her email address, require email verification of the new address

This feature, built into WordPress MS by default (when updating one's profile
via the Dashboard), ensures that new email addresses are valid and not
mistyped, helping to avoid unintentionally locked-out accounts.

Fixes #2265

Props j.conti for an initial patch

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8560 cdf35c40-ae34-48e0-9cc9-0c9da1808c22