Convert SpeakerVoucher to a generic Voucher issue system

[marcoacierno]

We have multiple places where we need to issue vouchers: speakers, co-speakers and grants. Those categories often overlap, e.g. a grant can also be a speaker, and we don't want to issue 2 vouchers to them.

The idea is to update our current speaker voucher system SpeakerVoucher to a more generic name ConferenceVoucher (WIP name) and extend the allowed types to have grant