Add infrastructure overview to T01 slides

A consistent problem I'm seeing is that students unfamiliar with git, GitLab, and/or mills get confused with the differences between everything and where all their stuff lives. I think it'd be good to have a slide that goes over:

• git: it's just a tool, it is not where your stuff lives
• GitLab: where our copy of your stuff lives
• mills: one place where you can keep a copy of your stuff, nobody else will see this
• your computer: another place where you can keep a copy of your stuff, nobody else will see this
• etc/expanded