Create an Ubuntu version of workspaces/xtermjs

[jonatanschroeder]

Given that the Desktop workspace and C/Java graders (as well as the new vscode workspace) are all using Ubuntu, I think it makes sense to migrate the xtermjs workspace to Ubuntu as well, or at least to create a new xtermjs-ubuntu image. It would also make sense to take that upgrade as a chance to add a few basic commands that are not included in this image, such as manpages, less, git, a more user-friendly text editor (e.g., nano), and basic compilers/environments like python and gcc/clang/valgrind.