Send a validation email when user's change their email address from the dashboard.

[ideadude]

This has been requested by at least 1 user.

The core WP software sends validation emails when a user (including non-admin users) changes their email from the edit profile page in the admin dashboard.

We should do the same kind of validation (at least as an option) so users don't feel their user management is less secure once LifterLMS is activated.

As a first pass, we would should copy the core WP functionality. Then we can consider having this as an option on the email address block of the edit user profile form.