Isabelle development self-hosted on Phorge

The Phorge project has succeeded Phabricator as a community fork, starting in June 2021 when the latter was “winding down” (see also https://sketis.net/2021/after-phabricator-we-phorge-it). In Dec-2023 Phorge has become sufficiently stable to consider moving the isabelle-dev service to it, using up-to-date versions of PHP and Ubuntu-Linux. Everything has been cleaned-up and reworked on our side, and we can continue this approach to self-hosting for more years to come.