Bug: Registration broken if multiple users signed in.

This [change](https://github.com/lynndylanhurley/devise_token_auth/pull/1075/files#diff-160c1afc120d57b344db17e0b7fc4280L16) made it so that there is no sign in when a user confirms their email address. Instead, `signed_in_resource` is called, which in turn calls warden authenticate.

This means that the redirect url does not send the proper params for authenticating with the redirect URL.

This can be fixed simply by adding back in the call to `sign_in(@resource)` and saving the resource after calling `create_token` on it a few lines down.

Open the linked change and trace the email-confirmation path through signed_in_resource and warden authenticate. Verify that restoring sign_in(@resource) and saving after create_token preserves the redirect URL's authentication parameters.

Written by the indexing model from the issue text.