Add ignore_period option to .confirm

[porras]

Hi and thank you for all your work, I've been using Devise in all my projects since forever, but this is my first contribution.

In most if not all projects, I end up needing this for an admin panel or backoffice, where an admin can mark an email as confirmed manually. Calling .confirm will not work if the period is expired. I know it is possible to resend the confirmation link and so reset the period, but for some reason or other, this is a flow support teams end up needing. It's very easy to implement of course, but I think it'd be nice to have it in the library, instead of going to the database behind the library's back.