autogenerate AUTHORS file to get rid of outdated data

The file can be re-generated using scripts/update-authors.sh.
File .mailmap is used for name canonicalization and fresh list of
authors replaces the old one in AUTHORS file automatically.
Gitlab CI checks its content before release (on branches named release-*).

Marek Vavruša is left there twice intentionally, once as CZ.NIC employee
and second time as external contributor.