Updating outdated screenshots and installation methods for GitLab

[borivojetasovac]

I noticed a few outdated GitLab screenshots in the 4.8 chapter ("Git on the Server - GitLab") of the book. Also, I'd like to update the installation guidelines to match the current best practices.

Any feedback/suggestions before I start?

[ben]

This would be great! I know GitLab has been shipping like crazy, so it's no surprise that section is outdated.

[HonkingGoose]

I've updated the text covering GitLab with #1504, but we still need newer GitLab pictures. I don't run a GitLab server, so I cannot take the pictures we need.

Can somebody take pictures and add them in a new pull-request that closes this issue? Thanks! 👍