use human-readable project IDs instead of numerical IDs in projects paths

[benjaminstrasser]

I cannot help myself, but I am not a big fan of having a numerical id in the path.
I would rather propose that when creating the project, the name property will be lowercased and whitespaces replaced by dashes. That way, we get a human readable id which we can use in the path.
Of course, this needs to be unique.

Originally posted by @mledl in #84 (comment)