Make compatible with current Core 4.0
As far as I see, for compatibility with the current core, the whole additional fields feature needs to be ported to Paweljong itself due to its removal in AlekSIS-Core. Issue: #58 (closed)
Apart from that, all apps that Paweljong depends on should be already compatible with the current Core. This needs testing.
Generally, it seems reasonable to me to rewrite the frontend based on Vue.js now.