Issue #368: Added 'Cancel' link to User forms.

Addresses https://github.com/backdrop/backdrop-issues/issues/368.
It looks like this doesn't happen for users adding a new account, but I can't figure out why. It be better to at least add a permissions check here, for `elseif (user_access('administer users'))` before using 'admin/people', then redirect to `<front>` as a worse-case scenario.