Tweaks confirmation flow for signed_in users

For #2627

When allow_unconfirmed_access_for > 0, users may
be already signed in at the time they confirm
their account. Consequently, the default
confirmation should be compatible with this
possibility. Additionally, they should not be
redirected to the sign in form after confirmation
in this case. So I've changed
ConfirmationsController#after_confirmation_path_for
to send the user to the root path when signed in,
or the sign in form otherwise.
Merge pull request #2634 from gregates/bug/confirmation_workflow

Tweaks confirmation flow for signed_in users