[Feature] Add an (optional) confirmation page on email confirmation of user

[sir-gawain]

We are using sf_register to allow people from anywhere to register an account for uploading there own projects for our events. After registration, the system sends the confirmation email with accept and refuse links. This works well for the majority of users, but for some people, it doesnt. They see the error message that the (seconds ago) created account is not known to the system.

We tracked it down to the email provider (in most of our cases MS Office 365). Some virus scanner software at the email provider checks links in emails, whether or not they are redirecting to malicious websites. With Office 365 this can be enabled by an organization policy and can not be disabled by the user. And this function instantly triggers the (new) account deletion in the confirmation email. At the point, where the user receives the mail, the account is already deleted and hence the message "unknown to the system".

I think, the simplest way to prevent this, is to add a new action with a confirmation to account removal.

[garbast]

You are welcome to provide a PR for this. But be aware, that this needs to cover accept, refuse, confirm and deny these are the actions that modify the state of the entity via link.