After more AI bot attacks: isabelle-dev.sketis.net is now for members only

[Makarius]

Recently we have seen massive attacks by dumb AI bots again. After tinkering 
with the webserver configuration for 1-2h, I did not manage to achieve 
significant improvements.

As a consequence, I have now disabled the public access to 
isabelle-dev.sketis.net --- it is for members only, who also need to be logged-in.

People who are directly involved in the isabelle-dev process and don't have an 
account yet can email me privately.


	Makarius