Forbid update of email address to one of another user

Ultimately, in the user information ("contact information") form, forbid
updating a user's email address to an email address in use by another user.
Because email addresses are a unique identifier of user accounts, they
cannot be reused between different users.
Merge pull request #129 from greezybacon/issue/validate-user-email-update

Forbid update of email address to one of another user

Reviewed-By: Peter Rotich <peter@osticket.com>