Shell filesystem consistency

In preparation for teaching the shell lessons at a bootcamp next week, I did some organizing of the material and wanted to make a pull request in case it would be useful to everyone else.

This PR puts all the material for the shell lessons into one consistent filetree, so that learners can be working form the same home directory. It also updates the lesson text and images so that everything should *hopefully* match the new layout. Unfortunately, you can't add empty directories to a git repository, so I removed them from the find lesson (this is part personal preference, as I don't think the -empty parameter is that useful to them at this stage).

Another personal preference change I made was removing the -print parameter from all of the find commands as we never explain it, and I think the less flags we introduce the better. Happy to be over-ruled on that one if someone has a good reason for keeping it in.

Whilst the lesson material is all in `filesystem/users/nelle` I've also added the file tree from the challenges to the first lesson into `filesystem/users` (although here again all the directories have to have a file inside to add them to the repo).

This might be relevent to the discussion in issue #646