Prompted by: https://matrix.org/blog/2019/11/09/avoiding-unwelcome-visitors-on-private-matrix-servers

This is a bit controversial, because.. the Synapse default remains open,
while the general advice (as per the blog post) is to make it more private.

I'm not sure exactly what kind of server people set up and whether they
want to make the room directory public. Our general goal is to favor
privacy and security when running personal (family & friends) and corporate
homeservers, both of which likely benefit from having a more secure default.