Let presenter confirm their attendance through the web UI

[kittykat]

Rather than waiting for each presenter to reply by email and confirm their attendance manually, the presenter should be able to confirm through https://registration.guadec.org/papers/list/own after their talk has been accepted.

For example, if the UI shows

Status: accepted

then there should be a button or something next to it saying "Confirm that you are coming" which should change the status to:

Status: accepted, attendance confirmed

Or something along those lines