I just tried to install Webmin on a fresh install of Ubuntu 18.04 Server in the same way as I did on 16.04. Unfortunately it failed on the command
apt install webmin
because the package apt-show-versions was not found. I solved this by adding the universe repository to apt with
sudo add-apt-repository universe
After this change it worked as expected.
Just in case somebody else is also missing the text editor e3 in Ubuntu 18.04 Server: It’s also in the universe repository.